diff --git a/core/lib/Drupal/Component/Attribute/Attribute.php b/core/lib/Drupal/Component/Attribute/Attribute.php
index 532ad9d..0d2db3b 100644
--- a/core/lib/Drupal/Component/Attribute/Attribute.php
+++ b/core/lib/Drupal/Component/Attribute/Attribute.php
@@ -317,7 +317,7 @@ public function toArray() {
   /**
    * Implements the magic __clone() method.
    */
-  public function  __clone() {
+  public function __clone() {
     foreach ($this->storage as $name => $value) {
       $this->storage[$name] = clone $value;
     }
diff --git a/core/lib/Drupal/Component/Attribute/AttributeArray.php b/core/lib/Drupal/Component/Attribute/AttributeArray.php
index 25bec95..085a0aa 100644
--- a/core/lib/Drupal/Component/Attribute/AttributeArray.php
+++ b/core/lib/Drupal/Component/Attribute/AttributeArray.php
@@ -89,13 +89,13 @@ public function getIterator() {
   /**
    * Exchange the array for another one.
    *
-   * @see ArrayObject::exchangeArray
-   *
    * @param array $input
    *   The array input to replace the internal value.
    *
    * @return array
    *   The old array value.
+   *
+   * @see ArrayObject::exchangeArray
    */
   public function exchangeArray($input) {
     $old = $this->value;
diff --git a/core/lib/Drupal/Component/Attribute/AttributeValueBase.php b/core/lib/Drupal/Component/Attribute/AttributeValueBase.php
index 1163db2..0b17627 100644
--- a/core/lib/Drupal/Component/Attribute/AttributeValueBase.php
+++ b/core/lib/Drupal/Component/Attribute/AttributeValueBase.php
@@ -70,6 +70,6 @@ public function value() {
   /**
    * Implements the magic __toString() method.
    */
-  abstract function __toString();
+  public abstract function __toString();
 
 }
diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index 23f63d6..83a9a7c 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -10,7 +10,9 @@
 use Drupal\Component\Attribute\Attribute as ComponentAttribute;
 
 /**
+ * Collects, sanitizes, and renders HTML attributes.
+ *
  * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
  *   Use \Drupal\Component\Attribute\Attribute.
  */
-class Attribute extends ComponentAttribute { }
+class Attribute extends ComponentAttribute {}
diff --git a/core/lib/Drupal/Core/Template/AttributeArray.php b/core/lib/Drupal/Core/Template/AttributeArray.php
index 9af0058..fb36fdf 100644
--- a/core/lib/Drupal/Core/Template/AttributeArray.php
+++ b/core/lib/Drupal/Core/Template/AttributeArray.php
@@ -10,7 +10,9 @@
 use Drupal\Component\Attribute\AttributeArray as ComponentAttributeArray;
 
 /**
+ * A class that defines a type of Attribute that can be added to as an array.
+ *
  * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
  *   Use \Drupal\Component\Attribute\AttributeArray.
  */
-class AttributeArray extends ComponentAttributeArray { }
+class AttributeArray extends ComponentAttributeArray {}
diff --git a/core/lib/Drupal/Core/Template/AttributeBoolean.php b/core/lib/Drupal/Core/Template/AttributeBoolean.php
index e7993e9..b03d130 100644
--- a/core/lib/Drupal/Core/Template/AttributeBoolean.php
+++ b/core/lib/Drupal/Core/Template/AttributeBoolean.php
@@ -10,7 +10,9 @@
 use Drupal\Component\Attribute\AttributeBoolean as ComponentAttributeBoolean;
 
 /**
+ * A class that defines a type of boolean HTML attribute.
+ *
  * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
  *   Use \Drupal\Component\Attribute\AttributeBoolean.
  */
-class AttributeBoolean extends ComponentAttributeBoolean { }
+class AttributeBoolean extends ComponentAttributeBoolean {}
diff --git a/core/lib/Drupal/Core/Template/AttributeString.php b/core/lib/Drupal/Core/Template/AttributeString.php
index c5bdedc..75445a2 100644
--- a/core/lib/Drupal/Core/Template/AttributeString.php
+++ b/core/lib/Drupal/Core/Template/AttributeString.php
@@ -10,7 +10,9 @@
 use Drupal\Component\Attribute\AttributeString as ComponentAttributeString;
 
 /**
+ * A class that represents most standard HTML attributes.
+ *
  * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
  *   Use \Drupal\Component\Attribute\AttributeString.
  */
-class AttributeString extends ComponentAttributeString { }
+class AttributeString extends ComponentAttributeString {}
diff --git a/core/lib/Drupal/Core/Template/AttributeValueBase.php b/core/lib/Drupal/Core/Template/AttributeValueBase.php
index 50407b7..68f142b 100644
--- a/core/lib/Drupal/Core/Template/AttributeValueBase.php
+++ b/core/lib/Drupal/Core/Template/AttributeValueBase.php
@@ -10,7 +10,9 @@
 use Drupal\Component\Attribute\AttributeValueBase as ComponentAttributeValueBase;
 
 /**
+ * Defines the base class for an attribute type.
+ *
  * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
  *   Use \Drupal\Component\Attribute\AttributeValueBase.
  */
-abstract class AttributeValueBase extends ComponentAttributeValueBase { }
+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
index 8f1379c..16d47a2 100644
--- a/core/tests/Drupal/Tests/Component/Attribute/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Component/Attribute/AttributeTest.php
@@ -79,6 +79,7 @@ public function testRemove() {
 
   /**
    * Tests setting attributes.
+   *
    * @covers ::setAttribute
    */
   public function testSetAttribute() {
@@ -107,6 +108,7 @@ public function testSetAttribute() {
 
   /**
    * Tests removing attributes.
+   *
    * @covers ::removeAttribute
    */
   public function testRemoveAttribute() {
@@ -146,6 +148,7 @@ public function testRemoveAttribute() {
 
   /**
    * Tests adding class attributes with the AttributeArray helper method.
+   *
    * @covers ::addClass
    */
   public function testAddClasses() {
@@ -189,20 +192,30 @@ public function testAddClasses() {
 
     // Add an array of classes.
     $attribute->addClass(array('red', 'green', 'blue'));
-    $this->assertArrayEquals(array('banana', 'aa', 'xx', 'yy', 'red', 'green', 'blue'), $attribute['class']->value());
+    $this->assertArrayEquals(
+      array('banana', 'aa', 'xx', 'yy', 'red', 'green', 'blue'),
+      $attribute['class']->value()
+    );
 
     // Add an array of duplicate classes.
-    $attribute->addClass(array('red', 'green', 'blue'), array('aa', 'aa', 'banana'), 'yy');
+    $attribute->addClass(
+      array('red', 'green', 'blue'),
+      array('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 = array('example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue', 'red');
+    $classes = array(
+      'example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue', 'red',
+    );
     $attribute = new Attribute(array('class' => $classes));
 
     // Remove one class.
@@ -229,6 +242,7 @@ public function testRemoveClasses() {
 
   /**
    * Tests checking for class names with the Attribute method.
+   *
    * @covers ::hasClass
    */
   public function testHasClass() {
@@ -244,6 +258,7 @@ public function testHasClass() {
 
   /**
    * Tests removing class attributes with the Attribute helper methods.
+   *
    * @covers ::removeClass
    * @covers ::addClass
    */
@@ -256,12 +271,15 @@ public function testChainAddRemoveClasses() {
       ->removeClass(array('red', 'green', 'pink'))
       ->addClass(array('apple', 'lime', 'grapefruit'))
       ->addClass(array('banana'));
-    $expected = array('example-class', 'blue', 'apple', 'lime', 'grapefruit', 'banana');
+    $expected = array(
+      '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
@@ -276,7 +294,7 @@ public function testTwigAddRemoveClasses($template, $expected, $seed_attributes
   }
 
   /**
-   * Provides tests data for testEscaping
+   * Provides tests data for testEscaping.
    *
    * @return array
    *   An array of test data each containing of a twig template string,
@@ -342,27 +360,40 @@ public function testIterate() {
    * Tests printing of an attribute.
    */
   public function testPrint() {
-    $attribute = new Attribute(array('class' => array('example-class'), 'id' => 'example-id', 'enabled' => TRUE));
+    $attribute = new Attribute(array(
+      'class' => array('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->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 = [];
 
@@ -386,7 +417,7 @@ public function providerTestAttributeValues() {
    */
   protected function assertClass($class, $html) {
     $xpath = "//*[@class='$class']";
-    self::assertTrue((bool) $this->getXPathResultCount($xpath, $html));
+    self::assertTrue((bool) $this->getXpathResultCount($xpath, $html));
   }
 
   /**
@@ -399,7 +430,7 @@ protected function assertClass($class, $html) {
    */
   protected function assertNoClass($class, $html) {
     $xpath = "//*[@class='$class']";
-    self::assertFalse((bool) $this->getXPathResultCount($xpath, $html));
+    self::assertFalse((bool) $this->getXpathResultCount($xpath, $html));
   }
 
   /**
@@ -410,9 +441,9 @@ protected function assertNoClass($class, $html) {
    * @param string $html
    *   The HTML snippet to check.
    */
-  protected function assertID($id, $html) {
+  protected function assertId($id, $html) {
     $xpath = "//*[@id='$id']";
-    self::assertTrue((bool) $this->getXPathResultCount($xpath, $html));
+    self::assertTrue((bool) $this->getXpathResultCount($xpath, $html));
   }
 
   /**
@@ -423,9 +454,9 @@ protected function assertID($id, $html) {
    * @param string $html
    *   The HTML snippet to check.
    */
-  protected function assertNoID($id, $html) {
+  protected function assertNoId($id, $html) {
     $xpath = "//*[@id='$id']";
-    self::assertFalse((bool) $this->getXPathResultCount($xpath, $html));
+    self::assertFalse((bool) $this->getXpathResultCount($xpath, $html));
   }
 
   /**
@@ -439,8 +470,8 @@ protected function assertNoID($id, $html) {
    * @return int
    *   The number of results that are found.
    */
-  protected function getXPathResultCount($query, $html) {
-    $document = new \DOMDocument;
+  protected function getXpathResultCount($query, $html) {
+    $document = new \DOMDocument();
     $document->loadHTML($html);
     $xpath = new \DOMXPath($document);
 
@@ -458,6 +489,10 @@ public function testStorage() {
 
 }
 
+/**
+ * Implementation of MarkupInterface to use in tests.
+ */
 class TestMarkup implements MarkupInterface, \Countable {
   use MarkupTrait;
+
 }