diff --git a/core/lib/Drupal/Component/Attribute/Attribute.php b/core/lib/Drupal/Component/Attribute/Attribute.php
new file mode 100644
index 0000000..d83c4ac
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/Attribute.php
@@ -0,0 +1,345 @@
+<?php
+
+namespace Drupal\Component\Attribute;
+
+use Drupal\Component\Render\PlainTextOutput;
+use Drupal\Component\Render\MarkupInterface;
+
+/**
+ * Collects, sanitizes, and renders HTML attributes.
+ *
+ * To use, optionally pass in an associative array of defined attributes, or
+ * add attributes using array syntax. For example:
+ * @code
+ *  $attributes = new Attribute(array('id' => 'socks'));
+ *  $attributes['class'] = array('black-cat', 'white-cat');
+ *  $attributes['class'][] = 'black-white-cat';
+ *  echo '<cat' . $attributes . '>';
+ *  // Produces <cat id="socks" class="black-cat white-cat black-white-cat">
+ * @endcode
+ *
+ * $attributes always prints out all the attributes. For example:
+ * @code
+ *  $attributes = new Attribute(array('id' => 'socks'));
+ *  $attributes['class'] = array('black-cat', 'white-cat');
+ *  $attributes['class'][] = 'black-white-cat';
+ *  echo '<cat class="cat ' . $attributes['class'] . '"' . $attributes . '>';
+ *  // Produces <cat class="cat black-cat white-cat black-white-cat" id="socks" class="cat black-cat white-cat black-white-cat">
+ * @endcode
+ *
+ * When printing out individual attributes to customize them within a Twig
+ * template, use the "without" filter to prevent attributes that have already
+ * been printed from being printed again. For example:
+ * @code
+ *  <cat class="{{ attributes.class }} my-custom-class"{{ attributes|without('class') }}>
+ *  {# Produces <cat class="cat black-cat white-cat black-white-cat my-custom-class" id="socks"> #}
+ * @endcode
+ *
+ * The attribute keys and values are automatically escaped for output with
+ * Html::escape(). No protocol filtering is applied, so when using user-entered
+ * input as a value for an attribute that expects an URI (href, src, ...),
+ * UrlHelper::stripDangerousProtocols() should be used to ensure dangerous
+ * protocols (such as 'javascript:') are removed. For example:
+ * @code
+ *  $path = 'javascript:alert("xss");';
+ *  $path = UrlHelper::stripDangerousProtocols($path);
+ *  $attributes = new Attribute(array('href' => $path));
+ *  echo '<a' . $attributes . '>';
+ *  // Produces <a href="alert(&quot;xss&quot;);">
+ * @endcode
+ *
+ * The attribute values are considered plain text and are treated as such. If a
+ * safe HTML string is detected, it is converted to plain text with
+ * PlainTextOutput::renderFromHtml() before being escaped. For example:
+ * @code
+ *   $value = t('Highlight the @tag tag', ['@tag' => '<em>']);
+ *   $attributes = new Attribute(['value' => $value]);
+ *   echo '<input' . $attributes . '>';
+ *   // Produces <input value="Highlight the &lt;em&gt; tag">
+ * @endcode
+ *
+ * @see \Drupal\Component\Utility\Html::escape()
+ * @see \Drupal\Component\Render\PlainTextOutput::renderFromHtml()
+ * @see \Drupal\Component\Utility\UrlHelper::stripDangerousProtocols()
+ */
+class Attribute implements \ArrayAccess, \IteratorAggregate, MarkupInterface {
+
+  /**
+   * Stores the attribute data.
+   *
+   * @var \Drupal\Component\Attribute\AttributeValueBase[]
+   */
+  protected $storage = array();
+
+  /**
+   * Constructs a \Drupal\Component\Attribute\Attribute object.
+   *
+   * @param array $attributes
+   *   An associative array of key-value pairs to be converted to attributes.
+   */
+  public function __construct($attributes = array()) {
+    foreach ($attributes as $name => $value) {
+      $this->offsetSet($name, $value);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetGet($name) {
+    if (isset($this->storage[$name])) {
+      return $this->storage[$name];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetSet($name, $value) {
+    $this->storage[$name] = $this->createAttributeValue($name, $value);
+  }
+
+  /**
+   * Creates the different types of attribute values.
+   *
+   * @param string $name
+   *   The attribute name.
+   * @param mixed $value
+   *   The attribute value.
+   *
+   * @return \Drupal\Component\Attribute\AttributeValueBase
+   *   An AttributeValueBase representation of the attribute's value.
+   */
+  protected function createAttributeValue($name, $value) {
+    // If the value is already an AttributeValueBase object,
+    // return a new instance of the same class, but with the new name.
+    if ($value instanceof AttributeValueBase) {
+      $class = get_class($value);
+      return new $class($name, $value->value());
+    }
+    // An array value or 'class' attribute name are forced to always be an
+    // AttributeArray value for consistency.
+    if ($name == 'class' && !is_array($value)) {
+      // Cast the value to string in case it implements MarkupInterface.
+      $value = [(string) $value];
+    }
+    if (is_array($value)) {
+      // Cast the value to an array if the value was passed in as a string.
+      // @todo Decide to fix all the broken instances of class as a string
+      // in core or cast them.
+      $value = new AttributeArray($name, $value);
+    }
+    elseif (is_bool($value)) {
+      $value = new AttributeBoolean($name, $value);
+    }
+    // As a development aid, we allow the value to be a safe string object.
+    elseif ($value instanceof MarkupInterface) {
+      // Attributes are not supposed to display HTML markup, so we just convert
+      // the value to plain text.
+      $value = PlainTextOutput::renderFromHtml($value);
+      $value = new AttributeString($name, $value);
+    }
+    elseif (!is_object($value)) {
+      $value = new AttributeString($name, $value);
+    }
+    return $value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetUnset($name) {
+    unset($this->storage[$name]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetExists($name) {
+    return isset($this->storage[$name]);
+  }
+
+  /**
+   * Adds classes or merges them on to array of existing CSS classes.
+   *
+   * @param string|array ...
+   *   CSS classes to add to the class attribute array.
+   *
+   * @return $this
+   */
+  public function addClass() {
+    $args = func_get_args();
+    if ($args) {
+      $classes = array();
+      foreach ($args as $arg) {
+        // Merge the values passed in from the classes array.
+        // The argument is cast to an array to support comma separated single
+        // values or one or more array arguments.
+        $classes = array_merge($classes, (array) $arg);
+      }
+
+      // Merge if there are values, just add them otherwise.
+      if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
+        // Merge the values passed in from the class value array.
+        $classes = array_merge($this->storage['class']->value(), $classes);
+        $this->storage['class']->exchangeArray($classes);
+      }
+      else {
+        $this->offsetSet('class', $classes);
+      }
+    }
+
+    return $this;
+  }
+
+  /**
+   * Sets values for an attribute key.
+   *
+   * @param string $attribute
+   *   Name of the attribute.
+   * @param string|array $value
+   *   Value(s) to set for the given attribute key.
+   *
+   * @return $this
+   */
+  public function setAttribute($attribute, $value) {
+    $this->offsetSet($attribute, $value);
+
+    return $this;
+  }
+
+  /**
+   * Removes an attribute from an Attribute object.
+   *
+   * @param string|array ...
+   *   Attributes to remove from the attribute array.
+   *
+   * @return $this
+   */
+  public function removeAttribute() {
+    $args = func_get_args();
+    foreach ($args as $arg) {
+      // Support arrays or multiple arguments.
+      if (is_array($arg)) {
+        foreach ($arg as $value) {
+          unset($this->storage[$value]);
+        }
+      }
+      else {
+        unset($this->storage[$arg]);
+      }
+    }
+
+    return $this;
+  }
+
+  /**
+   * Removes argument values from array of existing CSS classes.
+   *
+   * @param string|array ...
+   *   CSS classes to remove from the class attribute array.
+   *
+   * @return $this
+   */
+  public function removeClass() {
+    // With no class attribute, there is no need to remove.
+    if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
+      $args = func_get_args();
+      $classes = array();
+      foreach ($args as $arg) {
+        // Merge the values passed in from the classes array.
+        // The argument is cast to an array to support comma separated single
+        // values or one or more array arguments.
+        $classes = array_merge($classes, (array) $arg);
+      }
+
+      // Remove the values passed in from the value array. Use array_values() to
+      // ensure that the array index remains sequential.
+      $classes = array_values(array_diff($this->storage['class']->value(), $classes));
+      $this->storage['class']->exchangeArray($classes);
+    }
+    return $this;
+  }
+
+  /**
+   * Checks if the class array has the given CSS class.
+   *
+   * @param string $class
+   *   The CSS class to check for.
+   *
+   * @return bool
+   *   Returns TRUE if the class exists, or FALSE otherwise.
+   */
+  public function hasClass($class) {
+    if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
+      return in_array($class, $this->storage['class']->value());
+    }
+    else {
+      return FALSE;
+    }
+  }
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public function __toString() {
+    $return = '';
+    /** @var \Drupal\Component\Attribute\AttributeValueBase $value */
+    foreach ($this->storage as $name => $value) {
+      $rendered = $value->render();
+      if ($rendered) {
+        $return .= ' ' . $rendered;
+      }
+    }
+    return $return;
+  }
+
+  /**
+   * Returns all storage elements as an array.
+   *
+   * @return array
+   *   An associative array of attributes.
+   */
+  public function toArray() {
+    $return = [];
+    foreach ($this->storage as $name => $value) {
+      $return[$name] = $value->value();
+    }
+
+    return $return;
+  }
+
+  /**
+   * Implements the magic __clone() method.
+   */
+  public function __clone() {
+    foreach ($this->storage as $name => $value) {
+      $this->storage[$name] = clone $value;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIterator() {
+    return new \ArrayIterator($this->storage);
+  }
+
+  /**
+   * Returns the whole array.
+   */
+  public function storage() {
+    return $this->storage;
+  }
+
+  /**
+   * Returns a representation of the object for use in JSON serialization.
+   *
+   * @return string
+   *   The safe string content.
+   */
+  public function jsonSerialize() {
+    return (string) $this;
+  }
+
+}
diff --git a/core/lib/Drupal/Component/Attribute/AttributeArray.php b/core/lib/Drupal/Component/Attribute/AttributeArray.php
new file mode 100644
index 0000000..ad0d5d6
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/AttributeArray.php
@@ -0,0 +1,101 @@
+<?php
+
+namespace Drupal\Component\Attribute;
+
+use Drupal\Component\Utility\Html;
+
+/**
+ * A class that defines a type of Attribute that can be added to as an array.
+ *
+ * To use with Attribute, the array must be specified.
+ * Correct:
+ * @code
+ *  $attributes = new Attribute();
+ *  $attributes['class'] = array();
+ *  $attributes['class'][] = 'cat';
+ * @endcode
+ * Incorrect:
+ * @code
+ *  $attributes = new Attribute();
+ *  $attributes['class'][] = 'cat';
+ * @endcode
+ *
+ * @see \Drupal\Component\Attribute\Attribute
+ */
+class AttributeArray extends AttributeValueBase implements \ArrayAccess, \IteratorAggregate {
+
+  /**
+   * Ensures empty array as a result of array_filter will not print '$name=""'.
+   *
+   * @see \Drupal\Component\Attribute\AttributeArray::__toString()
+   * @see \Drupal\Component\Attribute\AttributeValueBase::render()
+   */
+  const RENDER_EMPTY_ATTRIBUTE = FALSE;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetGet($offset) {
+    return $this->value[$offset];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetSet($offset, $value) {
+    if (isset($offset)) {
+      $this->value[$offset] = $value;
+    }
+    else {
+      $this->value[] = $value;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetUnset($offset) {
+    unset($this->value[$offset]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function offsetExists($offset) {
+    return isset($this->value[$offset]);
+  }
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public function __toString() {
+    // Filter out any empty values before printing.
+    $this->value = array_unique(array_filter($this->value));
+    return Html::escape(implode(' ', $this->value));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIterator() {
+    return new \ArrayIterator($this->value);
+  }
+
+  /**
+   * Exchange the array for another one.
+   *
+   * @param array $input
+   *   The array input to replace the internal value.
+   *
+   * @return array
+   *   The old array value.
+   *
+   * @see ArrayObject::exchangeArray
+   */
+  public function exchangeArray($input) {
+    $old = $this->value;
+    $this->value = $input;
+    return $old;
+  }
+
+}
diff --git a/core/lib/Drupal/Component/Attribute/AttributeBoolean.php b/core/lib/Drupal/Component/Attribute/AttributeBoolean.php
new file mode 100644
index 0000000..d2cf928
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/AttributeBoolean.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\Component\Attribute;
+
+use Drupal\Component\Utility\Html;
+
+/**
+ * A class that defines a type of boolean HTML attribute.
+ *
+ * Boolean HTML attributes are not attributes with values of TRUE/FALSE.
+ * They are attributes that if they exist in the tag, they are TRUE.
+ * Examples include selected, disabled, checked, readonly.
+ *
+ * To set a boolean attribute on the Attribute class, set it to TRUE.
+ * @code
+ *  $attributes = new Attribute();
+ *  $attributes['disabled'] = TRUE;
+ *  echo '<select' . $attributes . '/>';
+ *  // produces <select disabled>;
+ *  $attributes['disabled'] = FALSE;
+ *  echo '<select' . $attributes . '/>';
+ *  // produces <select>;
+ * @endcode
+ *
+ * @see \Drupal\Component\Attribute\Attribute
+ */
+class AttributeBoolean extends AttributeValueBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return $this->__toString();
+  }
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public function __toString() {
+    return $this->value === FALSE ? '' : Html::escape($this->name);
+  }
+
+}
diff --git a/core/lib/Drupal/Component/Attribute/AttributeString.php b/core/lib/Drupal/Component/Attribute/AttributeString.php
new file mode 100644
index 0000000..fc059f6
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/AttributeString.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\Component\Attribute;
+
+use Drupal\Component\Utility\Html;
+
+/**
+ * A class that represents most standard HTML attributes.
+ *
+ * To use with the Attribute class, set the key to be the attribute name
+ * and the value the attribute value.
+ * @code
+ *  $attributes = new Attribute(array());
+ *  $attributes['id'] = 'socks';
+ *  $attributes['style'] = 'background-color:white';
+ *  echo '<cat ' . $attributes . '>';
+ *  // Produces: <cat id="socks" style="background-color:white">.
+ * @endcode
+ *
+ * @see \Drupal\Component\Attribute\Attribute
+ */
+class AttributeString extends AttributeValueBase {
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public function __toString() {
+    return Html::escape($this->value);
+  }
+
+}
diff --git a/core/lib/Drupal/Component/Attribute/AttributeValueBase.php b/core/lib/Drupal/Component/Attribute/AttributeValueBase.php
new file mode 100644
index 0000000..b05bb19
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/AttributeValueBase.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace Drupal\Component\Attribute;
+use Drupal\Component\Utility\Html;
+
+/**
+ * Defines the base class for an attribute type.
+ *
+ * @see \Drupal\Component\Attribute\Attribute
+ */
+abstract class AttributeValueBase {
+
+  /**
+   * Renders '$name=""' if $value is an empty string.
+   *
+   * @see \Drupal\Component\Attribute\AttributeValueBase::render()
+   */
+  const RENDER_EMPTY_ATTRIBUTE = TRUE;
+
+  /**
+   * The value itself.
+   *
+   * @var mixed
+   */
+  protected $value;
+
+  /**
+   * The name of the value.
+   *
+   * @var mixed
+   */
+  protected $name;
+
+  /**
+   * Constructs a \Drupal\Component\Attribute\AttributeValueBase object.
+   */
+  public function __construct($name, $value) {
+    $this->name = $name;
+    $this->value = $value;
+  }
+
+  /**
+   * Returns a string representation of the attribute.
+   *
+   * While __toString only returns the value in a string form, render()
+   * contains the name of the attribute as well.
+   *
+   * @return string
+   *   The string representation of the attribute.
+   */
+  public function render() {
+    $value = (string) $this;
+    if (isset($this->value) && static::RENDER_EMPTY_ATTRIBUTE || !empty($value)) {
+      return Html::escape($this->name) . '="' . $value . '"';
+    }
+  }
+
+  /**
+   * Returns the raw value.
+   */
+  public function value() {
+    return $this->value;
+  }
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public abstract function __toString();
+
+}
diff --git a/core/lib/Drupal/Component/Attribute/LICENSE.txt b/core/lib/Drupal/Component/Attribute/LICENSE.txt
new file mode 100644
index 0000000..94fb846
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/LICENSE.txt
@@ -0,0 +1,339 @@
+        GNU GENERAL PUBLIC LICENSE
+           Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+          Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software--to make sure the software is free for all its users.  This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it.  (Some other Free Software Foundation software is covered by
+the GNU Lesser General Public License instead.)  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+  To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have.  You must make sure that they, too, receive or can get the
+source code.  And you must show them these terms so they know their
+rights.
+
+  We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
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+
+  Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
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+  The precise terms and conditions for copying, distribution and
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+
+        GNU GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License applies to any program or other work which contains
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+Activities other than copying, distribution and modification are not
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+You may charge a fee for the physical act of transferring a copy, and
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+These requirements apply to the modified work as a whole.  If
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+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+    a) Accompany it with the complete corresponding machine-readable
+    source code, which must be distributed under the terms of Sections
+    1 and 2 above on a medium customarily used for software interchange; or,
+
+    b) Accompany it with a written offer, valid for at least three
+    years, to give any third party, for a charge no more than your
+    cost of physically performing source distribution, a complete
+    machine-readable copy of the corresponding source code, to be
+    distributed under the terms of Sections 1 and 2 above on a medium
+    customarily used for software interchange; or,
+
+    c) Accompany it with the information you received as to the offer
+    to distribute corresponding source code.  (This alternative is
+    allowed only for noncommercial distribution and only if you
+    received the program in object code or executable form with such
+    an offer, in accord with Subsection b above.)
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+The source code for a work means the preferred form of the work for
+making modifications to it.  For an executable work, complete source
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+associated interface definition files, plus the scripts used to
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+operating system on which the executable runs, unless that component
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+
+If distribution of executable or object code is made by offering
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+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  4. You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License.  Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+  5. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Program or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+  6. Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
+this License.
+
+  7. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Program at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Program by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
+apply and the section as a whole is intended to apply in other
+circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system, which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  8. If the distribution and/or use of the Program is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
+those countries, so that distribution is permitted only in or among
+countries not thus excluded.  In such case, this License incorporates
+the limitation as if written in the body of this License.
+
+  9. The Free Software Foundation may publish revised and/or new versions
+of the General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Program
+specifies a version number of this License which applies to it and "any
+later version", you have the option of following the terms and conditions
+either of that version or of any later version published by the Free
+Software Foundation.  If the Program does not specify a version number of
+this License, you may choose any version ever published by the Free Software
+Foundation.
+
+  10. If you wish to incorporate parts of the Program into other free
+programs whose distribution conditions are different, write to the author
+to ask for permission.  For software which is copyrighted by the Free
+Software Foundation, write to the Free Software Foundation; we sometimes
+make exceptions for this.  Our decision will be guided by the two goals
+of preserving the free status of all derivatives of our free software and
+of promoting the sharing and reuse of software generally.
+
+          NO WARRANTY
+
+  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
+PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
+OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
+REPAIR OR CORRECTION.
+
+  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
+REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
+INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
+OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
+TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
+YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
+PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGES.
+
+         END OF TERMS AND CONDITIONS
+
+      How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+    Gnomovision version 69, Copyright (C) year name of author
+    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, the commands you use may
+be called something other than `show w' and `show c'; they could even be
+mouse-clicks or menu items--whatever suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the program, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+  `Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+  <signature of Ty Coon>, 1 April 1989
+  Ty Coon, President of Vice
+
+This General Public License does not permit incorporating your program into
+proprietary programs.  If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.
diff --git a/core/lib/Drupal/Component/Attribute/README.txt b/core/lib/Drupal/Component/Attribute/README.txt
new file mode 100644
index 0000000..8ab8ede
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/README.txt
@@ -0,0 +1,12 @@
+The Drupal Attribute Component
+
+Thanks for using this Drupal component.
+
+You can participate in its development on Drupal.org, through our issue system:
+https://www.drupal.org/project/issues/drupal
+
+You can get the full Drupal repo here:
+https://www.drupal.org/project/drupal/git-instructions
+
+You can browse the full Drupal repo here:
+http://cgit.drupalcode.org/drupal
diff --git a/core/lib/Drupal/Component/Attribute/TESTING.txt b/core/lib/Drupal/Component/Attribute/TESTING.txt
new file mode 100644
index 0000000..96650d5
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/TESTING.txt
@@ -0,0 +1,18 @@
+HOW-TO: Test this Drupal component
+
+In order to test this component, you'll need to get the entire Drupal repo and
+run the tests there.
+
+You'll find the tests under core/tests/Drupal/Tests/Component.
+
+You can get the full Drupal repo here:
+https://www.drupal.org/project/drupal/git-instructions
+
+You can find more information about running PHPUnit tests with Drupal here:
+https://www.drupal.org/node/2116263
+
+Each component in the Drupal\Component namespace has its own annotated test
+group. You can use this group to run only the tests for this component. Like
+this:
+
+$ ./vendor/bin/phpunit -c core --group Attribute
diff --git a/core/lib/Drupal/Component/Attribute/composer.json b/core/lib/Drupal/Component/Attribute/composer.json
new file mode 100644
index 0000000..37f594e
--- /dev/null
+++ b/core/lib/Drupal/Component/Attribute/composer.json
@@ -0,0 +1,17 @@
+{
+    "name": "drupal/core-attribute",
+    "description": "Collects, sanitizes, and renders HTML attributes.",
+    "keywords": ["drupal"],
+    "homepage": "https://www.drupal.org/project/drupal",
+    "license": "GPL-2.0+",
+    "require": {
+        "php": ">=5.5.9",
+        "drupal/core-render": "~8.2",
+        "drupal/core-utility": "~8.2"
+    },
+    "autoload": {
+        "psr-4": {
+            "Drupal\\Component\\Attribute\\": ""
+        }
+    }
+}
diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index 5449cde..f2b167b 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -2,344 +2,12 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Render\PlainTextOutput;
-use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Attribute\Attribute as ComponentAttribute;
 
 /**
  * Collects, sanitizes, and renders HTML attributes.
  *
- * To use, optionally pass in an associative array of defined attributes, or
- * add attributes using array syntax. For example:
- * @code
- *  $attributes = new Attribute(array('id' => 'socks'));
- *  $attributes['class'] = array('black-cat', 'white-cat');
- *  $attributes['class'][] = 'black-white-cat';
- *  echo '<cat' . $attributes . '>';
- *  // Produces <cat id="socks" class="black-cat white-cat black-white-cat">
- * @endcode
- *
- * $attributes always prints out all the attributes. For example:
- * @code
- *  $attributes = new Attribute(array('id' => 'socks'));
- *  $attributes['class'] = array('black-cat', 'white-cat');
- *  $attributes['class'][] = 'black-white-cat';
- *  echo '<cat class="cat ' . $attributes['class'] . '"' . $attributes . '>';
- *  // Produces <cat class="cat black-cat white-cat black-white-cat" id="socks" class="cat black-cat white-cat black-white-cat">
- * @endcode
- *
- * When printing out individual attributes to customize them within a Twig
- * template, use the "without" filter to prevent attributes that have already
- * been printed from being printed again. For example:
- * @code
- *  <cat class="{{ attributes.class }} my-custom-class"{{ attributes|without('class') }}>
- *  {# Produces <cat class="cat black-cat white-cat black-white-cat my-custom-class" id="socks"> #}
- * @endcode
- *
- * The attribute keys and values are automatically escaped for output with
- * Html::escape(). No protocol filtering is applied, so when using user-entered
- * input as a value for an attribute that expects an URI (href, src, ...),
- * UrlHelper::stripDangerousProtocols() should be used to ensure dangerous
- * protocols (such as 'javascript:') are removed. For example:
- * @code
- *  $path = 'javascript:alert("xss");';
- *  $path = UrlHelper::stripDangerousProtocols($path);
- *  $attributes = new Attribute(array('href' => $path));
- *  echo '<a' . $attributes . '>';
- *  // Produces <a href="alert(&quot;xss&quot;);">
- * @endcode
- *
- * The attribute values are considered plain text and are treated as such. If a
- * safe HTML string is detected, it is converted to plain text with
- * PlainTextOutput::renderFromHtml() before being escaped. For example:
- * @code
- *   $value = t('Highlight the @tag tag', ['@tag' => '<em>']);
- *   $attributes = new Attribute(['value' => $value]);
- *   echo '<input' . $attributes . '>';
- *   // Produces <input value="Highlight the &lt;em&gt; tag">
- * @endcode
- *
- * @see \Drupal\Component\Utility\Html::escape()
- * @see \Drupal\Component\Render\PlainTextOutput::renderFromHtml()
- * @see \Drupal\Component\Utility\UrlHelper::stripDangerousProtocols()
+ * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
+ *   Use \Drupal\Component\Attribute\Attribute.
  */
-class Attribute implements \ArrayAccess, \IteratorAggregate, MarkupInterface {
-
-  /**
-   * Stores the attribute data.
-   *
-   * @var \Drupal\Core\Template\AttributeValueBase[]
-   */
-  protected $storage = [];
-
-  /**
-   * Constructs a \Drupal\Core\Template\Attribute object.
-   *
-   * @param array $attributes
-   *   An associative array of key-value pairs to be converted to attributes.
-   */
-  public function __construct($attributes = []) {
-    foreach ($attributes as $name => $value) {
-      $this->offsetSet($name, $value);
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetGet($name) {
-    if (isset($this->storage[$name])) {
-      return $this->storage[$name];
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetSet($name, $value) {
-    $this->storage[$name] = $this->createAttributeValue($name, $value);
-  }
-
-  /**
-   * Creates the different types of attribute values.
-   *
-   * @param string $name
-   *   The attribute name.
-   * @param mixed $value
-   *   The attribute value.
-   *
-   * @return \Drupal\Core\Template\AttributeValueBase
-   *   An AttributeValueBase representation of the attribute's value.
-   */
-  protected function createAttributeValue($name, $value) {
-    // If the value is already an AttributeValueBase object,
-    // return a new instance of the same class, but with the new name.
-    if ($value instanceof AttributeValueBase) {
-      $class = get_class($value);
-      return new $class($name, $value->value());
-    }
-    // An array value or 'class' attribute name are forced to always be an
-    // AttributeArray value for consistency.
-    if ($name == 'class' && !is_array($value)) {
-      // Cast the value to string in case it implements MarkupInterface.
-      $value = [(string) $value];
-    }
-    if (is_array($value)) {
-      // Cast the value to an array if the value was passed in as a string.
-      // @todo Decide to fix all the broken instances of class as a string
-      // in core or cast them.
-      $value = new AttributeArray($name, $value);
-    }
-    elseif (is_bool($value)) {
-      $value = new AttributeBoolean($name, $value);
-    }
-    // As a development aid, we allow the value to be a safe string object.
-    elseif ($value instanceof MarkupInterface) {
-      // Attributes are not supposed to display HTML markup, so we just convert
-      // the value to plain text.
-      $value = PlainTextOutput::renderFromHtml($value);
-      $value = new AttributeString($name, $value);
-    }
-    elseif (!is_object($value)) {
-      $value = new AttributeString($name, $value);
-    }
-    return $value;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetUnset($name) {
-    unset($this->storage[$name]);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetExists($name) {
-    return isset($this->storage[$name]);
-  }
-
-  /**
-   * Adds classes or merges them on to array of existing CSS classes.
-   *
-   * @param string|array ...
-   *   CSS classes to add to the class attribute array.
-   *
-   * @return $this
-   */
-  public function addClass() {
-    $args = func_get_args();
-    if ($args) {
-      $classes = [];
-      foreach ($args as $arg) {
-        // Merge the values passed in from the classes array.
-        // The argument is cast to an array to support comma separated single
-        // values or one or more array arguments.
-        $classes = array_merge($classes, (array) $arg);
-      }
-
-      // Merge if there are values, just add them otherwise.
-      if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
-        // Merge the values passed in from the class value array.
-        $classes = array_merge($this->storage['class']->value(), $classes);
-        $this->storage['class']->exchangeArray($classes);
-      }
-      else {
-        $this->offsetSet('class', $classes);
-      }
-    }
-
-    return $this;
-  }
-
-  /**
-   * Sets values for an attribute key.
-   *
-   * @param string $attribute
-   *   Name of the attribute.
-   * @param string|array $value
-   *   Value(s) to set for the given attribute key.
-   *
-   * @return $this
-   */
-  public function setAttribute($attribute, $value) {
-    $this->offsetSet($attribute, $value);
-
-    return $this;
-  }
-
-  /**
-   * Removes an attribute from an Attribute object.
-   *
-   * @param string|array ...
-   *   Attributes to remove from the attribute array.
-   *
-   * @return $this
-   */
-  public function removeAttribute() {
-    $args = func_get_args();
-    foreach ($args as $arg) {
-      // Support arrays or multiple arguments.
-      if (is_array($arg)) {
-        foreach ($arg as $value) {
-          unset($this->storage[$value]);
-        }
-      }
-      else {
-        unset($this->storage[$arg]);
-      }
-    }
-
-    return $this;
-  }
-
-  /**
-   * Removes argument values from array of existing CSS classes.
-   *
-   * @param string|array ...
-   *   CSS classes to remove from the class attribute array.
-   *
-   * @return $this
-   */
-  public function removeClass() {
-    // With no class attribute, there is no need to remove.
-    if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
-      $args = func_get_args();
-      $classes = [];
-      foreach ($args as $arg) {
-        // Merge the values passed in from the classes array.
-        // The argument is cast to an array to support comma separated single
-        // values or one or more array arguments.
-        $classes = array_merge($classes, (array) $arg);
-      }
-
-      // Remove the values passed in from the value array. Use array_values() to
-      // ensure that the array index remains sequential.
-      $classes = array_values(array_diff($this->storage['class']->value(), $classes));
-      $this->storage['class']->exchangeArray($classes);
-    }
-    return $this;
-  }
-
-  /**
-   * Checks if the class array has the given CSS class.
-   *
-   * @param string $class
-   *   The CSS class to check for.
-   *
-   * @return bool
-   *   Returns TRUE if the class exists, or FALSE otherwise.
-   */
-  public function hasClass($class) {
-    if (isset($this->storage['class']) && $this->storage['class'] instanceof AttributeArray) {
-      return in_array($class, $this->storage['class']->value());
-    }
-    else {
-      return FALSE;
-    }
-  }
-
-  /**
-   * Implements the magic __toString() method.
-   */
-  public function __toString() {
-    $return = '';
-    /** @var \Drupal\Core\Template\AttributeValueBase $value */
-    foreach ($this->storage as $name => $value) {
-      $rendered = $value->render();
-      if ($rendered) {
-        $return .= ' ' . $rendered;
-      }
-    }
-    return $return;
-  }
-
-  /**
-   * Returns all storage elements as an array.
-   *
-   * @return array
-   *   An associative array of attributes.
-   */
-  public function toArray() {
-    $return = [];
-    foreach ($this->storage as $name => $value) {
-      $return[$name] = $value->value();
-    }
-
-    return $return;
-  }
-
-  /**
-   * Implements the magic __clone() method.
-   */
-  public function __clone() {
-    foreach ($this->storage as $name => $value) {
-      $this->storage[$name] = clone $value;
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getIterator() {
-    return new \ArrayIterator($this->storage);
-  }
-
-  /**
-   * Returns the whole array.
-   */
-  public function storage() {
-    return $this->storage;
-  }
-
-  /**
-   * Returns a representation of the object for use in JSON serialization.
-   *
-   * @return string
-   *   The safe string content.
-   */
-  public function jsonSerialize() {
-    return (string) $this;
-  }
-
-}
+class Attribute extends ComponentAttribute {}
diff --git a/core/lib/Drupal/Core/Template/AttributeArray.php b/core/lib/Drupal/Core/Template/AttributeArray.php
index ce7186e..68465d2 100644
--- a/core/lib/Drupal/Core/Template/AttributeArray.php
+++ b/core/lib/Drupal/Core/Template/AttributeArray.php
@@ -2,100 +2,12 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Utility\Html;
+use Drupal\Component\Attribute\AttributeArray as ComponentAttributeArray;
 
 /**
  * A class that defines a type of Attribute that can be added to as an array.
  *
- * To use with Attribute, the array must be specified.
- * Correct:
- * @code
- *  $attributes = new Attribute();
- *  $attributes['class'] = array();
- *  $attributes['class'][] = 'cat';
- * @endcode
- * Incorrect:
- * @code
- *  $attributes = new Attribute();
- *  $attributes['class'][] = 'cat';
- * @endcode
- *
- * @see \Drupal\Core\Template\Attribute
+ * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
+ *   Use \Drupal\Component\Attribute\AttributeArray.
  */
-class AttributeArray extends AttributeValueBase implements \ArrayAccess, \IteratorAggregate {
-
-  /**
-   * Ensures empty array as a result of array_filter will not print '$name=""'.
-   *
-   * @see \Drupal\Core\Template\AttributeArray::__toString()
-   * @see \Drupal\Core\Template\AttributeValueBase::render()
-   */
-  const RENDER_EMPTY_ATTRIBUTE = FALSE;
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetGet($offset) {
-    return $this->value[$offset];
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetSet($offset, $value) {
-    if (isset($offset)) {
-      $this->value[$offset] = $value;
-    }
-    else {
-      $this->value[] = $value;
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetUnset($offset) {
-    unset($this->value[$offset]);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function offsetExists($offset) {
-    return isset($this->value[$offset]);
-  }
-
-  /**
-   * Implements the magic __toString() method.
-   */
-  public function __toString() {
-    // Filter out any empty values before printing.
-    $this->value = array_unique(array_filter($this->value));
-    return Html::escape(implode(' ', $this->value));
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getIterator() {
-    return new \ArrayIterator($this->value);
-  }
-
-  /**
-   * Exchange the array for another one.
-   *
-   * @see ArrayObject::exchangeArray
-   *
-   * @param array $input
-   *   The array input to replace the internal value.
-   *
-   * @return array
-   *   The old array value.
-   */
-  public function exchangeArray($input) {
-    $old = $this->value;
-    $this->value = $input;
-    return $old;
-  }
-
-}
+class AttributeArray extends ComponentAttributeArray {}
diff --git a/core/lib/Drupal/Core/Template/AttributeBoolean.php b/core/lib/Drupal/Core/Template/AttributeBoolean.php
index cc94820..f20aae0 100644
--- a/core/lib/Drupal/Core/Template/AttributeBoolean.php
+++ b/core/lib/Drupal/Core/Template/AttributeBoolean.php
@@ -2,42 +2,12 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Utility\Html;
+use Drupal\Component\Attribute\AttributeBoolean as ComponentAttributeBoolean;
 
 /**
  * A class that defines a type of boolean HTML attribute.
  *
- * Boolean HTML attributes are not attributes with values of TRUE/FALSE.
- * They are attributes that if they exist in the tag, they are TRUE.
- * Examples include selected, disabled, checked, readonly.
- *
- * To set a boolean attribute on the Attribute class, set it to TRUE.
- * @code
- *  $attributes = new Attribute();
- *  $attributes['disabled'] = TRUE;
- *  echo '<select' . $attributes . '/>';
- *  // produces <select disabled>;
- *  $attributes['disabled'] = FALSE;
- *  echo '<select' . $attributes . '/>';
- *  // produces <select>;
- * @endcode
- *
- * @see \Drupal\Core\Template\Attribute
+ * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
+ *   Use \Drupal\Component\Attribute\AttributeBoolean.
  */
-class AttributeBoolean extends AttributeValueBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  public function render() {
-    return $this->__toString();
-  }
-
-  /**
-   * Implements the magic __toString() method.
-   */
-  public function __toString() {
-    return $this->value === FALSE ? '' : Html::escape($this->name);
-  }
-
-}
+class AttributeBoolean extends ComponentAttributeBoolean {}
diff --git a/core/lib/Drupal/Core/Template/AttributeString.php b/core/lib/Drupal/Core/Template/AttributeString.php
index d4416bb..e2a34f2 100644
--- a/core/lib/Drupal/Core/Template/AttributeString.php
+++ b/core/lib/Drupal/Core/Template/AttributeString.php
@@ -2,30 +2,12 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Utility\Html;
+use Drupal\Component\Attribute\AttributeString as ComponentAttributeString;
 
 /**
  * A class that represents most standard HTML attributes.
  *
- * To use with the Attribute class, set the key to be the attribute name
- * and the value the attribute value.
- * @code
- *  $attributes = new Attribute(array());
- *  $attributes['id'] = 'socks';
- *  $attributes['style'] = 'background-color:white';
- *  echo '<cat ' . $attributes . '>';
- *  // Produces: <cat id="socks" style="background-color:white">.
- * @endcode
- *
- * @see \Drupal\Core\Template\Attribute
+ * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
+ *   Use \Drupal\Component\Attribute\AttributeString.
  */
-class AttributeString extends AttributeValueBase {
-
-  /**
-   * Implements the magic __toString() method.
-   */
-  public function __toString() {
-    return Html::escape($this->value);
-  }
-
-}
+class AttributeString extends ComponentAttributeString {}
diff --git a/core/lib/Drupal/Core/Template/AttributeValueBase.php b/core/lib/Drupal/Core/Template/AttributeValueBase.php
index 3dfdf4c..890a328 100644
--- a/core/lib/Drupal/Core/Template/AttributeValueBase.php
+++ b/core/lib/Drupal/Core/Template/AttributeValueBase.php
@@ -2,70 +2,12 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Utility\Html;
+use Drupal\Component\Attribute\AttributeValueBase as ComponentAttributeValueBase;
 
 /**
  * Defines the base class for an attribute type.
  *
- * @see \Drupal\Core\Template\Attribute
+ * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
+ *   Use \Drupal\Component\Attribute\AttributeValueBase.
  */
-abstract class AttributeValueBase {
-
-  /**
-   * Renders '$name=""' if $value is an empty string.
-   *
-   * @see \Drupal\Core\Template\AttributeValueBase::render()
-   */
-  const RENDER_EMPTY_ATTRIBUTE = TRUE;
-
-  /**
-   * The value itself.
-   *
-   * @var mixed
-   */
-  protected $value;
-
-  /**
-   * The name of the value.
-   *
-   * @var mixed
-   */
-  protected $name;
-
-  /**
-   * Constructs a \Drupal\Core\Template\AttributeValueBase object.
-   */
-  public function __construct($name, $value) {
-    $this->name = $name;
-    $this->value = $value;
-  }
-
-  /**
-   * Returns a string representation of the attribute.
-   *
-   * While __toString only returns the value in a string form, render()
-   * contains the name of the attribute as well.
-   *
-   * @return string
-   *   The string representation of the attribute.
-   */
-  public function render() {
-    $value = (string) $this;
-    if (isset($this->value) && static::RENDER_EMPTY_ATTRIBUTE || !empty($value)) {
-      return Html::escape($this->name) . '="' . $value . '"';
-    }
-  }
-
-  /**
-   * Returns the raw value.
-   */
-  public function value() {
-    return $this->value;
-  }
-
-  /**
-   * Implements the magic __toString() method.
-   */
-  abstract public function __toString();
-
-}
+abstract class AttributeValueBase extends ComponentAttributeValueBase {}
diff --git a/core/tests/Drupal/Tests/Component/Attribute/AttributeTest.php b/core/tests/Drupal/Tests/Component/Attribute/AttributeTest.php
new file mode 100644
index 0000000..c0dd76c
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Attribute/AttributeTest.php
@@ -0,0 +1,485 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Component\Attribute\AttributeTest.
+ */
+
+namespace Drupal\Tests\Component\Attribute;
+
+use Drupal\Component\Attribute\Attribute;
+use Drupal\Component\Attribute\AttributeArray;
+use Drupal\Component\Attribute\AttributeString;
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Render\MarkupTrait;
+use Drupal\Component\Utility\Html;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Component\Attribute\Attribute
+ * @group Attribute
+ */
+class AttributeTest extends UnitTestCase {
+
+  /**
+   * Tests the constructor of the attribute class.
+   */
+  public function testConstructor() {
+    $attribute = new Attribute(['class' => ['example-class']]);
+    $this->assertTrue(isset($attribute['class']));
+    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
+
+    // Test adding boolean attributes through the constructor.
+    $attribute = new Attribute(['selected' => TRUE, 'checked' => FALSE]);
+    $this->assertTrue($attribute['selected']->value());
+    $this->assertFalse($attribute['checked']->value());
+
+    // Test that non-array values with name "class" are cast to array.
+    $attribute = new Attribute(['class' => 'example-class']);
+    $this->assertTrue(isset($attribute['class']));
+    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
+
+    // Test that safe string objects work correctly.
+    $safe_string = $this->prophesize(MarkupInterface::class);
+    $safe_string->__toString()->willReturn('example-class');
+    $attribute = new Attribute(['class' => $safe_string->reveal()]);
+    $this->assertTrue(isset($attribute['class']));
+    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
+  }
+
+  /**
+   * Tests set of values.
+   */
+  public function testSet() {
+    $attribute = new Attribute();
+    $attribute['class'] = ['example-class'];
+
+    $this->assertTrue(isset($attribute['class']));
+    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
+  }
+
+  /**
+   * Tests adding new values to an existing part of the attribute.
+   */
+  public function testAdd() {
+    $attribute = new Attribute(['class' => ['example-class']]);
+
+    $attribute['class'][] = 'other-class';
+    $this->assertEquals(new AttributeArray('class', ['example-class', 'other-class']), $attribute['class']);
+  }
+
+  /**
+   * Tests removing of values.
+   */
+  public function testRemove() {
+    $attribute = new Attribute(['class' => ['example-class']]);
+    unset($attribute['class']);
+    $this->assertFalse(isset($attribute['class']));
+  }
+
+  /**
+   * Tests setting attributes.
+   *
+   * @covers ::setAttribute
+   */
+  public function testSetAttribute() {
+    $attribute = new Attribute();
+
+    // Test adding various attributes.
+    $attributes = ['alt', 'id', 'src', 'title', 'value'];
+    foreach ($attributes as $key) {
+      foreach (['kitten', ''] as $value) {
+        $attribute = new Attribute();
+        $attribute->setAttribute($key, $value);
+        $this->assertEquals($value, $attribute[$key]);
+      }
+    }
+
+    // Test adding array to class.
+    $attribute = new Attribute();
+    $attribute->setAttribute('class', ['kitten', 'cat']);
+    $this->assertArrayEquals(['kitten', 'cat'], $attribute['class']->value());
+
+    // Test adding boolean attributes.
+    $attribute = new Attribute();
+    $attribute['checked'] = TRUE;
+    $this->assertTrue($attribute['checked']->value());
+  }
+
+  /**
+   * Tests removing attributes.
+   *
+   * @covers ::removeAttribute
+   */
+  public function testRemoveAttribute() {
+    $attributes = [
+      'alt' => 'Alternative text',
+      'id' => 'bunny',
+      'src' => 'zebra',
+      'style' => 'color: pink;',
+      'title' => 'kitten',
+      'value' => 'ostrich',
+      'checked' => TRUE,
+    ];
+    $attribute = new Attribute($attributes);
+
+    // Single value.
+    $attribute->removeAttribute('alt');
+    $this->assertEmpty($attribute['alt']);
+
+    // Multiple values.
+    $attribute->removeAttribute('id', 'src');
+    $this->assertEmpty($attribute['id']);
+    $this->assertEmpty($attribute['src']);
+
+    // Single value in array.
+    $attribute->removeAttribute(['style']);
+    $this->assertEmpty($attribute['style']);
+
+    // Boolean value.
+    $attribute->removeAttribute('checked');
+    $this->assertEmpty($attribute['checked']);
+
+    // Multiple values in array.
+    $attribute->removeAttribute(['title', 'value']);
+    $this->assertEmpty((string) $attribute);
+
+  }
+
+  /**
+   * Tests adding class attributes with the AttributeArray helper method.
+   *
+   * @covers ::addClass
+   */
+  public function testAddClasses() {
+    // Add empty Attribute object with no classes.
+    $attribute = new Attribute();
+
+    // Add no class on empty attribute.
+    $attribute->addClass();
+    $this->assertEmpty($attribute['class']);
+
+    // Test various permutations of adding values to empty Attribute objects.
+    foreach ([NULL, FALSE, '', []] as $value) {
+      // Single value.
+      $attribute->addClass($value);
+      $this->assertEmpty((string) $attribute);
+
+      // Multiple values.
+      $attribute->addClass($value, $value);
+      $this->assertEmpty((string) $attribute);
+
+      // Single value in array.
+      $attribute->addClass([$value]);
+      $this->assertEmpty((string) $attribute);
+
+      // Single value in arrays.
+      $attribute->addClass([$value], [$value]);
+      $this->assertEmpty((string) $attribute);
+    }
+
+    // Add one class on empty attribute.
+    $attribute->addClass('banana');
+    $this->assertArrayEquals(['banana'], $attribute['class']->value());
+
+    // Add one class.
+    $attribute->addClass('aa');
+    $this->assertArrayEquals(['banana', 'aa'], $attribute['class']->value());
+
+    // Add multiple classes.
+    $attribute->addClass('xx', 'yy');
+    $this->assertArrayEquals(['banana', 'aa', 'xx', 'yy'], $attribute['class']->value());
+
+    // Add an array of classes.
+    $attribute->addClass(['red', 'green', 'blue']);
+    $this->assertArrayEquals(['banana', 'aa', 'xx', 'yy', 'red', 'green', 'blue'], $attribute['class']->value());
+
+    // Add an array of duplicate classes.
+    $attribute->addClass(['red', 'green', 'blue'], ['aa', 'aa', 'banana'], 'yy');
+    $this->assertEquals('banana aa xx yy red green blue', (string) $attribute['class']);
+  }
+
+  /**
+   * Tests removing class attributes with the AttributeArray helper method.
+   *
+   * @covers ::removeClass
+   */
+  public function testRemoveClasses() {
+    // Add duplicate class to ensure that both duplicates are removed.
+    $classes = ['example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue', 'red'];
+    $attribute = new Attribute(['class' => $classes]);
+
+    // Remove one class.
+    $attribute->removeClass('example-class');
+    $this->assertNotContains('example-class', $attribute['class']->value());
+
+    // Remove multiple classes.
+    $attribute->removeClass('xx', 'yy');
+    $this->assertNotContains(['xx', 'yy'], $attribute['class']->value());
+
+    // Remove an array of classes.
+    $attribute->removeClass(['red', 'green', 'blue']);
+    $this->assertNotContains(['red', 'green', 'blue'], $attribute['class']->value());
+
+    // Remove a class that does not exist.
+    $attribute->removeClass('gg');
+    $this->assertNotContains(['gg'], $attribute['class']->value());
+    // Test that the array index remains sequential.
+    $this->assertArrayEquals(['aa'], $attribute['class']->value());
+
+    $attribute->removeClass('aa');
+    $this->assertEmpty((string) $attribute);
+  }
+
+  /**
+   * Tests checking for class names with the Attribute method.
+   *
+   * @covers ::hasClass
+   */
+  public function testHasClass() {
+    // Test an attribute without any classes.
+    $attribute = new Attribute();
+    $this->assertFalse($attribute->hasClass('a-class-nowhere-to-be-found'));
+
+    // Add a class to check for.
+    $attribute->addClass('we-totally-have-this-class');
+    // Check that this class exists.
+    $this->assertTrue($attribute->hasClass('we-totally-have-this-class'));
+  }
+
+  /**
+   * Tests removing class attributes with the Attribute helper methods.
+   *
+   * @covers ::removeClass
+   * @covers ::addClass
+   */
+  public function testChainAddRemoveClasses() {
+    $attribute = new Attribute(
+      ['class' => ['example-class', 'red', 'green', 'blue']]
+    );
+
+    $attribute
+      ->removeClass(['red', 'green', 'pink'])
+      ->addClass(['apple', 'lime', 'grapefruit'])
+      ->addClass(['banana']);
+    $expected = ['example-class', 'blue', 'apple', 'lime', 'grapefruit', 'banana'];
+    $this->assertArrayEquals($expected, $attribute['class']->value(), 'Attributes chained');
+  }
+
+  /**
+   * Tests the twig calls to the Attribute.
+   *
+   * @dataProvider providerTestAttributeClassHelpers
+   *
+   * @covers ::removeClass
+   * @covers ::addClass
+   *
+   * @group legacy
+   */
+  public function testTwigAddRemoveClasses($template, $expected, $seed_attributes = []) {
+    $loader = new \Twig_Loader_String();
+    $twig = new \Twig_Environment($loader);
+    $data = ['attributes' => new Attribute($seed_attributes)];
+    $result = $twig->render($template, $data);
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * Provides tests data for testEscaping.
+   *
+   * @return array
+   *   An array of test data each containing of a twig template string,
+   *   a resulting string of classes and an optional array of attributes.
+   */
+  public function providerTestAttributeClassHelpers() {
+    return [
+      ["{{ attributes.class }}", ''],
+      ["{{ attributes.addClass('everest').class }}", 'everest'],
+      ["{{ attributes.addClass(['k2', 'kangchenjunga']).class }}", 'k2 kangchenjunga'],
+      ["{{ attributes.addClass('lhotse', 'makalu', 'cho-oyu').class }}", 'lhotse makalu cho-oyu'],
+      [
+        "{{ attributes.addClass('nanga-parbat').class }}",
+        'dhaulagiri manaslu nanga-parbat',
+        ['class' => ['dhaulagiri', 'manaslu']],
+      ],
+      [
+        "{{ attributes.removeClass('annapurna').class }}",
+        'gasherbrum-i',
+        ['class' => ['annapurna', 'gasherbrum-i']],
+      ],
+      [
+        "{{ attributes.removeClass(['broad peak']).class }}",
+        'gasherbrum-ii',
+        ['class' => ['broad peak', 'gasherbrum-ii']],
+      ],
+      [
+        "{{ attributes.removeClass('gyachung-kang', 'shishapangma').class }}",
+        '',
+        ['class' => ['shishapangma', 'gyachung-kang']],
+      ],
+      [
+        "{{ attributes.removeClass('nuptse').addClass('annapurna-ii').class }}",
+        'himalchuli annapurna-ii',
+        ['class' => ['himalchuli', 'nuptse']],
+      ],
+      // Test for the removal of an empty class name.
+      ["{{ attributes.addClass('rakaposhi', '').class }}", 'rakaposhi'],
+    ];
+  }
+
+  /**
+   * Tests iterating on the values of the attribute.
+   */
+  public function testIterate() {
+    $attribute = new Attribute(['class' => ['example-class'], 'id' => 'example-id']);
+
+    $counter = 0;
+    foreach ($attribute as $key => $value) {
+      if ($counter == 0) {
+        $this->assertEquals('class', $key);
+        $this->assertEquals(new AttributeArray('class', ['example-class']), $value);
+      }
+      if ($counter == 1) {
+        $this->assertEquals('id', $key);
+        $this->assertEquals(new AttributeString('id', 'example-id'), $value);
+      }
+      $counter++;
+    }
+  }
+
+  /**
+   * Tests printing of an attribute.
+   */
+  public function testPrint() {
+    $attribute = new Attribute(['class' => ['example-class'], 'id' => 'example-id', 'enabled' => TRUE]);
+
+    $content = $this->randomMachineName();
+    $html = '<div' . (string) $attribute . '>' . $content . '</div>';
+    $this->assertClass('example-class', $html);
+    $this->assertNoClass('example-class2', $html);
+
+    $this->assertId('example-id', $html);
+    $this->assertNoId('example-id2', $html);
+
+    $this->assertTrue(strpos($html, 'enabled') !== FALSE);
+  }
+
+  /**
+   * Tests attribute values.
+   *
+   * @covers ::createAttributeValue
+   *
+   * @dataProvider providerTestAttributeValues
+   */
+  public function testAttributeValues(array $attributes, $expected) {
+    $this->assertEquals($expected, (new Attribute($attributes))->__toString());
+  }
+
+  /**
+   * Provides test data for testAttributeValues.
+   *
+   * @return array
+   *   An array of test data.
+   */
+  public function providerTestAttributeValues() {
+    $data = [];
+
+    $string = '"> <script>alert(123)</script>"';
+    $data['safe-object-xss1'] = [['title' => TestMarkup::create($string)], ' title="&quot;&gt; alert(123)&quot;"'];
+    $data['non-safe-object-xss1'] = [['title' => $string], ' title="' . Html::escape($string) . '"'];
+    $string = '&quot;><script>alert(123)</script>';
+    $data['safe-object-xss2'] = [['title' => TestMarkup::create($string)], ' title="&quot;&gt;alert(123)"'];
+    $data['non-safe-object-xss2'] = [['title' => $string], ' title="' . Html::escape($string) . '"'];
+
+    return $data;
+  }
+
+  /**
+   * Checks that the given CSS class is present in the given HTML snippet.
+   *
+   * @param string $class
+   *   The CSS class to check.
+   * @param string $html
+   *   The HTML snippet to check.
+   */
+  protected function assertClass($class, $html) {
+    $xpath = "//*[@class='$class']";
+    self::assertTrue((bool) $this->getXpathResultCount($xpath, $html));
+  }
+
+  /**
+   * Checks that the given CSS class is not present in the given HTML snippet.
+   *
+   * @param string $class
+   *   The CSS class to check.
+   * @param string $html
+   *   The HTML snippet to check.
+   */
+  protected function assertNoClass($class, $html) {
+    $xpath = "//*[@class='$class']";
+    self::assertFalse((bool) $this->getXpathResultCount($xpath, $html));
+  }
+
+  /**
+   * Checks that the given CSS ID is present in the given HTML snippet.
+   *
+   * @param string $id
+   *   The CSS ID to check.
+   * @param string $html
+   *   The HTML snippet to check.
+   */
+  protected function assertId($id, $html) {
+    $xpath = "//*[@id='$id']";
+    self::assertTrue((bool) $this->getXpathResultCount($xpath, $html));
+  }
+
+  /**
+   * Checks that the given CSS ID is not present in the given HTML snippet.
+   *
+   * @param string $id
+   *   The CSS ID to check.
+   * @param string $html
+   *   The HTML snippet to check.
+   */
+  protected function assertNoId($id, $html) {
+    $xpath = "//*[@id='$id']";
+    self::assertFalse((bool) $this->getXpathResultCount($xpath, $html));
+  }
+
+  /**
+   * Counts the occurrences of the given XPath query in a given HTML snippet.
+   *
+   * @param string $query
+   *   The XPath query to execute.
+   * @param string $html
+   *   The HTML snippet to check.
+   *
+   * @return int
+   *   The number of results that are found.
+   */
+  protected function getXpathResultCount($query, $html) {
+    $document = new \DOMDocument();
+    $document->loadHTML($html);
+    $xpath = new \DOMXPath($document);
+
+    return $xpath->query($query)->length;
+  }
+
+  /**
+   * Tests the storage method.
+   */
+  public function testStorage() {
+    $attribute = new Attribute(['class' => ['example-class']]);
+
+    $this->assertEquals(['class' => new AttributeArray('class', ['example-class'])], $attribute->storage());
+  }
+
+}
+
+/**
+ * Implementation of MarkupInterface to use in tests.
+ */
+class TestMarkup implements MarkupInterface, \Countable {
+  use MarkupTrait;
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
deleted file mode 100644
index c9bccce..0000000
--- a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ /dev/null
@@ -1,456 +0,0 @@
-<?php
-
-namespace Drupal\Tests\Core\Template;
-
-use Drupal\Component\Utility\Html;
-use Drupal\Core\Render\Markup;
-use Drupal\Core\Template\Attribute;
-use Drupal\Core\Template\AttributeArray;
-use Drupal\Core\Template\AttributeString;
-use Drupal\Tests\UnitTestCase;
-use Drupal\Component\Render\MarkupInterface;
-
-/**
- * @coversDefaultClass \Drupal\Core\Template\Attribute
- * @group Template
- */
-class AttributeTest extends UnitTestCase {
-
-  /**
-   * Tests the constructor of the attribute class.
-   */
-  public function testConstructor() {
-    $attribute = new Attribute(['class' => ['example-class']]);
-    $this->assertTrue(isset($attribute['class']));
-    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
-
-    // Test adding boolean attributes through the constructor.
-    $attribute = new Attribute(['selected' => TRUE, 'checked' => FALSE]);
-    $this->assertTrue($attribute['selected']->value());
-    $this->assertFalse($attribute['checked']->value());
-
-    // Test that non-array values with name "class" are cast to array.
-    $attribute = new Attribute(['class' => 'example-class']);
-    $this->assertTrue(isset($attribute['class']));
-    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
-
-    // Test that safe string objects work correctly.
-    $safe_string = $this->prophesize(MarkupInterface::class);
-    $safe_string->__toString()->willReturn('example-class');
-    $attribute = new Attribute(['class' => $safe_string->reveal()]);
-    $this->assertTrue(isset($attribute['class']));
-    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
-  }
-
-  /**
-   * Tests set of values.
-   */
-  public function testSet() {
-    $attribute = new Attribute();
-    $attribute['class'] = ['example-class'];
-
-    $this->assertTrue(isset($attribute['class']));
-    $this->assertEquals(new AttributeArray('class', ['example-class']), $attribute['class']);
-  }
-
-  /**
-   * Tests adding new values to an existing part of the attribute.
-   */
-  public function testAdd() {
-    $attribute = new Attribute(['class' => ['example-class']]);
-
-    $attribute['class'][] = 'other-class';
-    $this->assertEquals(new AttributeArray('class', ['example-class', 'other-class']), $attribute['class']);
-  }
-
-  /**
-   * Tests removing of values.
-   */
-  public function testRemove() {
-    $attribute = new Attribute(['class' => ['example-class']]);
-    unset($attribute['class']);
-    $this->assertFalse(isset($attribute['class']));
-  }
-
-  /**
-   * Tests setting attributes.
-   * @covers ::setAttribute
-   */
-  public function testSetAttribute() {
-    $attribute = new Attribute();
-
-    // Test adding various attributes.
-    $attributes = ['alt', 'id', 'src', 'title', 'value'];
-    foreach ($attributes as $key) {
-      foreach (['kitten', ''] as $value) {
-        $attribute = new Attribute();
-        $attribute->setAttribute($key, $value);
-        $this->assertEquals($value, $attribute[$key]);
-      }
-    }
-
-    // Test adding array to class.
-    $attribute = new Attribute();
-    $attribute->setAttribute('class', ['kitten', 'cat']);
-    $this->assertArrayEquals(['kitten', 'cat'], $attribute['class']->value());
-
-    // Test adding boolean attributes.
-    $attribute = new Attribute();
-    $attribute['checked'] = TRUE;
-    $this->assertTrue($attribute['checked']->value());
-  }
-
-  /**
-   * Tests removing attributes.
-   * @covers ::removeAttribute
-   */
-  public function testRemoveAttribute() {
-    $attributes = [
-      'alt' => 'Alternative text',
-      'id' => 'bunny',
-      'src' => 'zebra',
-      'style' => 'color: pink;',
-      'title' => 'kitten',
-      'value' => 'ostrich',
-      'checked' => TRUE,
-    ];
-    $attribute = new Attribute($attributes);
-
-    // Single value.
-    $attribute->removeAttribute('alt');
-    $this->assertEmpty($attribute['alt']);
-
-    // Multiple values.
-    $attribute->removeAttribute('id', 'src');
-    $this->assertEmpty($attribute['id']);
-    $this->assertEmpty($attribute['src']);
-
-    // Single value in array.
-    $attribute->removeAttribute(['style']);
-    $this->assertEmpty($attribute['style']);
-
-    // Boolean value.
-    $attribute->removeAttribute('checked');
-    $this->assertEmpty($attribute['checked']);
-
-    // Multiple values in array.
-    $attribute->removeAttribute(['title', 'value']);
-    $this->assertEmpty((string) $attribute);
-
-  }
-
-  /**
-   * Tests adding class attributes with the AttributeArray helper method.
-   * @covers ::addClass
-   */
-  public function testAddClasses() {
-    // Add empty Attribute object with no classes.
-    $attribute = new Attribute();
-
-    // Add no class on empty attribute.
-    $attribute->addClass();
-    $this->assertEmpty($attribute['class']);
-
-    // Test various permutations of adding values to empty Attribute objects.
-    foreach ([NULL, FALSE, '', []] as $value) {
-      // Single value.
-      $attribute->addClass($value);
-      $this->assertEmpty((string) $attribute);
-
-      // Multiple values.
-      $attribute->addClass($value, $value);
-      $this->assertEmpty((string) $attribute);
-
-      // Single value in array.
-      $attribute->addClass([$value]);
-      $this->assertEmpty((string) $attribute);
-
-      // Single value in arrays.
-      $attribute->addClass([$value], [$value]);
-      $this->assertEmpty((string) $attribute);
-    }
-
-    // Add one class on empty attribute.
-    $attribute->addClass('banana');
-    $this->assertArrayEquals(['banana'], $attribute['class']->value());
-
-    // Add one class.
-    $attribute->addClass('aa');
-    $this->assertArrayEquals(['banana', 'aa'], $attribute['class']->value());
-
-    // Add multiple classes.
-    $attribute->addClass('xx', 'yy');
-    $this->assertArrayEquals(['banana', 'aa', 'xx', 'yy'], $attribute['class']->value());
-
-    // Add an array of classes.
-    $attribute->addClass(['red', 'green', 'blue']);
-    $this->assertArrayEquals(['banana', 'aa', 'xx', 'yy', 'red', 'green', 'blue'], $attribute['class']->value());
-
-    // Add an array of duplicate classes.
-    $attribute->addClass(['red', 'green', 'blue'], ['aa', 'aa', 'banana'], 'yy');
-    $this->assertEquals('banana aa xx yy red green blue', (string) $attribute['class']);
-  }
-
-  /**
-   * Tests removing class attributes with the AttributeArray helper method.
-   * @covers ::removeClass
-   */
-  public function testRemoveClasses() {
-    // Add duplicate class to ensure that both duplicates are removed.
-    $classes = ['example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue', 'red'];
-    $attribute = new Attribute(['class' => $classes]);
-
-    // Remove one class.
-    $attribute->removeClass('example-class');
-    $this->assertNotContains('example-class', $attribute['class']->value());
-
-    // Remove multiple classes.
-    $attribute->removeClass('xx', 'yy');
-    $this->assertNotContains(['xx', 'yy'], $attribute['class']->value());
-
-    // Remove an array of classes.
-    $attribute->removeClass(['red', 'green', 'blue']);
-    $this->assertNotContains(['red', 'green', 'blue'], $attribute['class']->value());
-
-    // Remove a class that does not exist.
-    $attribute->removeClass('gg');
-    $this->assertNotContains(['gg'], $attribute['class']->value());
-    // Test that the array index remains sequential.
-    $this->assertArrayEquals(['aa'], $attribute['class']->value());
-
-    $attribute->removeClass('aa');
-    $this->assertEmpty((string) $attribute);
-  }
-
-  /**
-   * Tests checking for class names with the Attribute method.
-   * @covers ::hasClass
-   */
-  public function testHasClass() {
-    // Test an attribute without any classes.
-    $attribute = new Attribute();
-    $this->assertFalse($attribute->hasClass('a-class-nowhere-to-be-found'));
-
-    // Add a class to check for.
-    $attribute->addClass('we-totally-have-this-class');
-    // Check that this class exists.
-    $this->assertTrue($attribute->hasClass('we-totally-have-this-class'));
-  }
-
-  /**
-   * Tests removing class attributes with the Attribute helper methods.
-   * @covers ::removeClass
-   * @covers ::addClass
-   */
-  public function testChainAddRemoveClasses() {
-    $attribute = new Attribute(
-      ['class' => ['example-class', 'red', 'green', 'blue']]
-    );
-
-    $attribute
-      ->removeClass(['red', 'green', 'pink'])
-      ->addClass(['apple', 'lime', 'grapefruit'])
-      ->addClass(['banana']);
-    $expected = ['example-class', 'blue', 'apple', 'lime', 'grapefruit', 'banana'];
-    $this->assertArrayEquals($expected, $attribute['class']->value(), 'Attributes chained');
-  }
-
-  /**
-   * Tests the twig calls to the Attribute.
-   * @dataProvider providerTestAttributeClassHelpers
-   *
-   * @covers ::removeClass
-   * @covers ::addClass
-   *
-   * @group legacy
-   */
-  public function testTwigAddRemoveClasses($template, $expected, $seed_attributes = []) {
-    $loader = new \Twig_Loader_String();
-    $twig = new \Twig_Environment($loader);
-    $data = ['attributes' => new Attribute($seed_attributes)];
-    $result = $twig->render($template, $data);
-    $this->assertEquals($expected, $result);
-  }
-
-  /**
-   * Provides tests data for testEscaping
-   *
-   * @return array
-   *   An array of test data each containing of a twig template string,
-   *   a resulting string of classes and an optional array of attributes.
-   */
-  public function providerTestAttributeClassHelpers() {
-    return [
-      ["{{ attributes.class }}", ''],
-      ["{{ attributes.addClass('everest').class }}", 'everest'],
-      ["{{ attributes.addClass(['k2', 'kangchenjunga']).class }}", 'k2 kangchenjunga'],
-      ["{{ attributes.addClass('lhotse', 'makalu', 'cho-oyu').class }}", 'lhotse makalu cho-oyu'],
-      [
-        "{{ attributes.addClass('nanga-parbat').class }}",
-        'dhaulagiri manaslu nanga-parbat',
-        ['class' => ['dhaulagiri', 'manaslu']],
-      ],
-      [
-        "{{ attributes.removeClass('annapurna').class }}",
-        'gasherbrum-i',
-        ['class' => ['annapurna', 'gasherbrum-i']],
-      ],
-      [
-        "{{ attributes.removeClass(['broad peak']).class }}",
-        'gasherbrum-ii',
-        ['class' => ['broad peak', 'gasherbrum-ii']],
-      ],
-      [
-        "{{ attributes.removeClass('gyachung-kang', 'shishapangma').class }}",
-        '',
-        ['class' => ['shishapangma', 'gyachung-kang']],
-      ],
-      [
-        "{{ attributes.removeClass('nuptse').addClass('annapurna-ii').class }}",
-        'himalchuli annapurna-ii',
-        ['class' => ['himalchuli', 'nuptse']],
-      ],
-      // Test for the removal of an empty class name.
-      ["{{ attributes.addClass('rakaposhi', '').class }}", 'rakaposhi'],
-    ];
-  }
-
-  /**
-   * Tests iterating on the values of the attribute.
-   */
-  public function testIterate() {
-    $attribute = new Attribute(['class' => ['example-class'], 'id' => 'example-id']);
-
-    $counter = 0;
-    foreach ($attribute as $key => $value) {
-      if ($counter == 0) {
-        $this->assertEquals('class', $key);
-        $this->assertEquals(new AttributeArray('class', ['example-class']), $value);
-      }
-      if ($counter == 1) {
-        $this->assertEquals('id', $key);
-        $this->assertEquals(new AttributeString('id', 'example-id'), $value);
-      }
-      $counter++;
-    }
-  }
-
-  /**
-   * Tests printing of an attribute.
-   */
-  public function testPrint() {
-    $attribute = new Attribute(['class' => ['example-class'], 'id' => 'example-id', 'enabled' => TRUE]);
-
-    $content = $this->randomMachineName();
-    $html = '<div' . (string) $attribute . '>' . $content . '</div>';
-    $this->assertClass('example-class', $html);
-    $this->assertNoClass('example-class2', $html);
-
-    $this->assertID('example-id', $html);
-    $this->assertNoID('example-id2', $html);
-
-    $this->assertTrue(strpos($html, 'enabled') !== FALSE);
-  }
-
-  /**
-   * @covers ::createAttributeValue
-   * @dataProvider providerTestAttributeValues
-   */
-  public function testAttributeValues(array $attributes, $expected) {
-    $this->assertEquals($expected, (new Attribute($attributes))->__toString());
-  }
-
-  public function providerTestAttributeValues() {
-    $data = [];
-
-    $string = '"> <script>alert(123)</script>"';
-    $data['safe-object-xss1'] = [['title' => Markup::create($string)], ' title="&quot;&gt; alert(123)&quot;"'];
-    $data['non-safe-object-xss1'] = [['title' => $string], ' title="' . Html::escape($string) . '"'];
-    $string = '&quot;><script>alert(123)</script>';
-    $data['safe-object-xss2'] = [['title' => Markup::create($string)], ' title="&quot;&gt;alert(123)"'];
-    $data['non-safe-object-xss2'] = [['title' => $string], ' title="' . Html::escape($string) . '"'];
-
-    return $data;
-  }
-
-  /**
-   * Checks that the given CSS class is present in the given HTML snippet.
-   *
-   * @param string $class
-   *   The CSS class to check.
-   * @param string $html
-   *   The HTML snippet to check.
-   */
-  protected function assertClass($class, $html) {
-    $xpath = "//*[@class='$class']";
-    self::assertTrue((bool) $this->getXPathResultCount($xpath, $html));
-  }
-
-  /**
-   * Checks that the given CSS class is not present in the given HTML snippet.
-   *
-   * @param string $class
-   *   The CSS class to check.
-   * @param string $html
-   *   The HTML snippet to check.
-   */
-  protected function assertNoClass($class, $html) {
-    $xpath = "//*[@class='$class']";
-    self::assertFalse((bool) $this->getXPathResultCount($xpath, $html));
-  }
-
-  /**
-   * Checks that the given CSS ID is present in the given HTML snippet.
-   *
-   * @param string $id
-   *   The CSS ID to check.
-   * @param string $html
-   *   The HTML snippet to check.
-   */
-  protected function assertID($id, $html) {
-    $xpath = "//*[@id='$id']";
-    self::assertTrue((bool) $this->getXPathResultCount($xpath, $html));
-  }
-
-  /**
-   * Checks that the given CSS ID is not present in the given HTML snippet.
-   *
-   * @param string $id
-   *   The CSS ID to check.
-   * @param string $html
-   *   The HTML snippet to check.
-   */
-  protected function assertNoID($id, $html) {
-    $xpath = "//*[@id='$id']";
-    self::assertFalse((bool) $this->getXPathResultCount($xpath, $html));
-  }
-
-  /**
-   * Counts the occurrences of the given XPath query in a given HTML snippet.
-   *
-   * @param string $query
-   *   The XPath query to execute.
-   * @param string $html
-   *   The HTML snippet to check.
-   *
-   * @return int
-   *   The number of results that are found.
-   */
-  protected function getXPathResultCount($query, $html) {
-    $document = new \DOMDocument();
-    $document->loadHTML($html);
-    $xpath = new \DOMXPath($document);
-
-    return $xpath->query($query)->length;
-  }
-
-  /**
-   * Tests the storage method.
-   */
-  public function testStorage() {
-    $attribute = new Attribute(['class' => ['example-class']]);
-
-    $this->assertEquals(['class' => new AttributeArray('class', ['example-class'])], $attribute->storage());
-  }
-
-}