diff --git a/core/modules/contact/src/Tests/ContactAuthenticatedUserTest.php b/core/modules/contact/tests/src/Functional/ContactAuthenticatedUserTest.php
similarity index 84%
rename from core/modules/contact/src/Tests/ContactAuthenticatedUserTest.php
rename to core/modules/contact/tests/src/Functional/ContactAuthenticatedUserTest.php
index eb7c527..4b4a520 100644
--- a/core/modules/contact/src/Tests/ContactAuthenticatedUserTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactAuthenticatedUserTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\contact\Tests;
+namespace Drupal\Tests\contact\Functional;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests contact form textfields are present if authenticated.
  *
  * @group contact
  */
-class ContactAuthenticatedUserTest extends WebTestBase {
+class ContactAuthenticatedUserTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/contact/src/Tests/ContactLanguageTest.php b/core/modules/contact/tests/src/Functional/ContactLanguageTest.php
similarity index 92%
rename from core/modules/contact/src/Tests/ContactLanguageTest.php
rename to core/modules/contact/tests/src/Functional/ContactLanguageTest.php
index 4623bda..47d1533 100644
--- a/core/modules/contact/src/Tests/ContactLanguageTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactLanguageTest.php
@@ -1,8 +1,8 @@
 <?php
 
-namespace Drupal\contact\Tests;
+namespace Drupal\Tests\contact\Functional;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests contact messages with language module.
@@ -13,7 +13,7 @@
  *
  * @group contact
  */
-class ContactLanguageTest extends WebTestBase {
+class ContactLanguageTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/contact/src/Tests/ContactPersonalTest.php b/core/modules/contact/tests/src/Functional/ContactPersonalTest.php
similarity index 97%
rename from core/modules/contact/src/Tests/ContactPersonalTest.php
rename to core/modules/contact/tests/src/Functional/ContactPersonalTest.php
index 936aed5..40be162 100644
--- a/core/modules/contact/src/Tests/ContactPersonalTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactPersonalTest.php
@@ -1,11 +1,13 @@
 <?php
 
-namespace Drupal\contact\Tests;
+namespace Drupal\Tests\contact\Functional;
 
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Render\PlainTextOutput;
 use Drupal\Core\Session\AccountInterface;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Core\Test\AssertMailTrait;
+use Drupal\system\Tests\Cache\AssertPageCacheContextsAndTagsTrait;
+use Drupal\Tests\BrowserTestBase;
 use Drupal\user\RoleInterface;
 
 /**
@@ -13,7 +15,10 @@
  *
  * @group contact
  */
-class ContactPersonalTest extends WebTestBase {
+class ContactPersonalTest extends BrowserTestBase {
+
+  use AssertMailTrait;
+  use AssertPageCacheContextsAndTagsTrait;
 
   /**
    * Modules to enable.
@@ -67,7 +72,7 @@ function testSendPersonalContactMessage() {
     $this->drupalGet('user/' . $this->contactUser->id() . '/contact');
     $this->assertEscaped($mail);
     $message = $this->submitPersonalContact($this->contactUser);
-    $mails = $this->drupalGetMails();
+    $mails = $this->getMails();
     $this->assertEqual(1, count($mails));
     $mail = $mails[0];
     $this->assertEqual($mail['to'], $this->contactUser->getEmail());
diff --git a/core/modules/contact/src/Tests/ContactSitewideTest.php b/core/modules/contact/tests/src/Functional/ContactSitewideTest.php
similarity index 97%
rename from core/modules/contact/src/Tests/ContactSitewideTest.php
rename to core/modules/contact/tests/src/Functional/ContactSitewideTest.php
index c148715..0f27086 100644
--- a/core/modules/contact/src/Tests/ContactSitewideTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactSitewideTest.php
@@ -1,13 +1,14 @@
 <?php
 
-namespace Drupal\contact\Tests;
+namespace Drupal\Tests\contact\Functional;
 
 use Drupal\Component\Utility\Unicode;
 use Drupal\contact\Entity\ContactForm;
 use Drupal\Core\Mail\MailFormatHelper;
+use Drupal\Core\Test\AssertMailTrait;
 use Drupal\Core\Url;
 use Drupal\field_ui\Tests\FieldUiTestTrait;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\user\RoleInterface;
 
@@ -18,9 +19,10 @@
  *
  * @group contact
  */
-class ContactSitewideTest extends WebTestBase {
+class ContactSitewideTest extends BrowserTestBase {
 
   use FieldUiTestTrait;
+  use AssertMailTrait;
 
   /**
    * Modules to enable.
@@ -265,8 +267,8 @@ function testSiteWideContact() {
 
     // Find out in which row the form we want to add a field to is.
     $i = 0;
-    foreach ($this->xpath('//table/tbody/tr') as $row) {
-      if (((string) $row->td[0]->a) == $label) {
+    foreach ($this->xpath('//table/tbody/tr/td[1]/a') as $row_link) {
+      if ($row_link->getText() == $label) {
         break;
       }
       $i++;
@@ -294,7 +296,7 @@ function testSiteWideContact() {
       $field_name . '[0][value]' => $this->randomMachineName(),
     );
     $this->drupalPostForm(NULL, $edit, t('Send message'));
-    $mails = $this->drupalGetMails();
+    $mails = $this->getMails();
     $mail = array_pop($mails);
     $this->assertEqual($mail['subject'], t('[@label] @subject', array('@label' => $label, '@subject' => $edit['subject[0][value]'])));
     $this->assertTrue(strpos($mail['body'], $field_label));
@@ -366,7 +368,7 @@ function testAutoReply() {
     $this->submitContact($this->randomMachineName(16), $email, $subject, 'foo', $this->randomString(128));
 
     // We are testing the auto-reply, so there should be one email going to the sender.
-    $captured_emails = $this->drupalGetMails(array('id' => 'contact_page_autoreply', 'to' => $email));
+    $captured_emails = $this->getMails(array('id' => 'contact_page_autoreply', 'to' => $email));
     $this->assertEqual(count($captured_emails), 1);
     $this->assertEqual(trim($captured_emails[0]['body']), trim(MailFormatHelper::htmlToText($foo_autoreply)));
 
@@ -375,14 +377,14 @@ function testAutoReply() {
     $this->submitContact($this->randomMachineName(16), $email, $this->randomString(64), 'bar', $this->randomString(128));
 
     // Auto-reply for form 'bar' should result in one auto-reply email to the sender.
-    $captured_emails = $this->drupalGetMails(array('id' => 'contact_page_autoreply', 'to' => $email));
+    $captured_emails = $this->getMails(array('id' => 'contact_page_autoreply', 'to' => $email));
     $this->assertEqual(count($captured_emails), 1);
     $this->assertEqual(trim($captured_emails[0]['body']), trim(MailFormatHelper::htmlToText($bar_autoreply)));
 
     // Verify that no auto-reply is sent when the auto-reply field is left blank.
     $email = $this->randomMachineName(32) . '@example.com';
     $this->submitContact($this->randomMachineName(16), $email, $this->randomString(64), 'no_autoreply', $this->randomString(128));
-    $captured_emails = $this->drupalGetMails(array('id' => 'contact_page_autoreply', 'to' => $email));
+    $captured_emails = $this->getMails(array('id' => 'contact_page_autoreply', 'to' => $email));
     $this->assertEqual(count($captured_emails), 0);
   }
 
diff --git a/core/modules/contact/src/Tests/ContactStorageTest.php b/core/modules/contact/tests/src/Functional/ContactStorageTest.php
similarity index 98%
rename from core/modules/contact/src/Tests/ContactStorageTest.php
rename to core/modules/contact/tests/src/Functional/ContactStorageTest.php
index ad0e8d4..f32233a 100644
--- a/core/modules/contact/src/Tests/ContactStorageTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactStorageTest.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\contact\Tests;
+namespace Drupal\Tests\contact\Functional;
 
 use Drupal\Component\Utility\Unicode;
 use Drupal\contact\Entity\Message;
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index da77a67..b0f90ed 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -25,6 +25,7 @@
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Test\AssertMailTrait;
 use Drupal\Core\Url;
+use Drupal\system\Tests\Cache\AssertPageCacheContextsAndTagsTrait;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\Yaml\Yaml as SymfonyYaml;
@@ -38,6 +39,7 @@
 abstract class WebTestBase extends TestBase {
 
   use AssertContentTrait;
+  use AssertPageCacheContextsAndTagsTrait;
   use BlockCreationTrait {
     placeBlock as drupalPlaceBlock;
   }
@@ -2708,28 +2710,6 @@ protected function buildUrl($path, array $options = array()) {
   }
 
   /**
-   * Asserts whether an expected cache context was present in the last response.
-   *
-   * @param string $expected_cache_context
-   *   The expected cache context.
-   */
-  protected function assertCacheContext($expected_cache_context) {
-    $cache_contexts = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Contexts'));
-    $this->assertTrue(in_array($expected_cache_context, $cache_contexts), "'" . $expected_cache_context . "' is present in the X-Drupal-Cache-Contexts header.");
-  }
-
-  /**
-   * Asserts that a cache context was not present in the last response.
-   *
-   * @param string $not_expected_cache_context
-   *   The expected cache context.
-   */
-  protected function assertNoCacheContext($not_expected_cache_context) {
-    $cache_contexts = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Contexts'));
-    $this->assertFalse(in_array($not_expected_cache_context, $cache_contexts), "'" . $not_expected_cache_context . "' is not present in the X-Drupal-Cache-Contexts header.");
-  }
-
-  /**
    * Asserts whether an expected cache tag was present in the last response.
    *
    * @param string $expected_cache_tag
diff --git a/core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php b/core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php
index 5c1e3f5..af72836 100644
--- a/core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php
+++ b/core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php
@@ -32,11 +32,19 @@ public function testGoTo() {
 
     // Test page contains some text.
     $this->assertSession()->pageTextContains('Test page text.');
+    // Make sure CSS <style> content is not considered as part of the page text.
+    $this->assertSession()->pageTextNotContains('@import');
+    // Make sure JS setting values are not part of the page text.
+    $this->assertSession()->pageTextNotContains('"baseUrl"');
 
     // Check that returned plain text is correct.
     $text = $this->getTextContent();
     $this->assertContains('Test page text.', $text);
     $this->assertNotContains('</html>', $text);
+    // No CSS styles in the plain text response.
+    $this->assertNotContains('@import', $text);
+    // No Drupal JS settings in the plain text response.
+    $this->assertNotContains('"baseUrl"', $text);
 
     // Response includes cache tags that we can assert.
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache-Tags', 'rendered');
diff --git a/core/modules/system/src/Tests/Cache/AssertPageCacheContextsAndTagsTrait.php b/core/modules/system/src/Tests/Cache/AssertPageCacheContextsAndTagsTrait.php
index 9864053..e7fd9cd 100644
--- a/core/modules/system/src/Tests/Cache/AssertPageCacheContextsAndTagsTrait.php
+++ b/core/modules/system/src/Tests/Cache/AssertPageCacheContextsAndTagsTrait.php
@@ -41,6 +41,28 @@ protected function getCacheHeaderValues($header_name) {
   }
 
   /**
+   * Asserts whether an expected cache context was present in the last response.
+   *
+   * @param string $expected_cache_context
+   *   The expected cache context.
+   */
+  protected function assertCacheContext($expected_cache_context) {
+    $cache_contexts = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Contexts'));
+    $this->assertTrue(in_array($expected_cache_context, $cache_contexts), "'" . $expected_cache_context . "' is present in the X-Drupal-Cache-Contexts header.");
+  }
+
+  /**
+   * Asserts that a cache context was not present in the last response.
+   *
+   * @param string $not_expected_cache_context
+   *   The expected cache context.
+   */
+  protected function assertNoCacheContext($not_expected_cache_context) {
+    $cache_contexts = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Contexts'));
+    $this->assertFalse(in_array($not_expected_cache_context, $cache_contexts), "'" . $not_expected_cache_context . "' is not present in the X-Drupal-Cache-Contexts header.");
+  }
+
+  /**
    * Asserts page cache miss, then hit for the given URL; checks cache headers.
    *
    * @param \Drupal\Core\Url $url
diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index 5785a7d..476919d 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -57,6 +57,9 @@ protected function assertElementNotPresent($css_selector) {
    *   $this->assertSession()->responseContains() instead.
    */
   protected function assertText($text) {
+    // Cast MarkupInterface to string.
+    $text = (string) $text;
+
     $content_type = $this->getSession()->getResponseHeader('Content-type');
     // In case of a Non-HTML response (example: XML) check the original
     // response.
@@ -64,6 +67,10 @@ protected function assertText($text) {
       $this->assertSession()->responseContains($text);
     }
     else {
+      // Mink/BrowserKit uses PHP's DOMElement to get the value of an HTML tag.
+      // It will provide the value HTML entity decoded, so we need to do the
+      // same here for the text to actually match.
+      $text = html_entity_decode($text);
       $this->assertSession()->pageTextContains($text);
     }
   }
@@ -82,6 +89,9 @@ protected function assertText($text) {
    *   $this->assertSession()->responseNotContains() instead.
    */
   protected function assertNoText($text) {
+    // Cast MarkupInterface to string.
+    $text = (string) $text;
+
     $content_type = $this->getSession()->getResponseHeader('Content-type');
     // In case of a Non-HTML response (example: XML) check the original
     // response.
@@ -89,6 +99,10 @@ protected function assertNoText($text) {
       $this->assertSession()->responseNotContains($text);
     }
     else {
+      // Mink/BrowserKit uses PHP's DOMElement to get the value of an HTML tag.
+      // It will provide the value HTML entity decoded, so we need to do the
+      // same here for the text to actually match.
+      $text = html_entity_decode($text);
       $this->assertSession()->pageTextNotContains($text);
     }
   }
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index c8df47c..767a834 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -5,7 +5,6 @@
 use Behat\Mink\Driver\GoutteDriver;
 use Behat\Mink\Element\Element;
 use Behat\Mink\Mink;
-use Behat\Mink\Session;
 use Drupal\Component\FileCache\FileCacheFactory;
 use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\Html;
diff --git a/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php b/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
index 00dbdf5..fb15bdb 100644
--- a/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
+++ b/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
@@ -6,6 +6,7 @@
 use Behat\Mink\Element\NodeElement;
 use Behat\Mink\Session;
 use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Utility\Html;
 use Drupal\FunctionalTests\AssertLegacyTrait;
 use Drupal\Tests\UnitTestCase;
 use Drupal\Tests\WebAssert;
@@ -154,6 +155,52 @@ public function testAssertOptionSelectedFail() {
   }
 
   /**
+   * @covers ::assertText
+   */
+  public function testAssertText() {
+    $dangerous = 'Bad html <script>alert(123);</script>';
+    $sanitized = Html::escape($dangerous);
+
+    $this->session
+      ->getResponseHeader('Content-type')
+      ->willReturn('text/html');
+    $this->webAssert
+      ->pageTextContains($sanitized)
+      ->shouldNotBeCalled();
+    $this->webAssert
+      ->pageTextContains($dangerous)
+      ->shouldBeCalled();
+
+    // The legacy ::assertText() returns nothing and calls no PHPUnit assertion,
+    // thus we need a real assertion. If ::assertText() fails, then it will
+    // return the exception and this test will fail.
+    $this->assertNull($this->assertText($sanitized));
+  }
+
+  /**
+   * @covers ::assertNoText
+   */
+  public function testAssertNoText() {
+    $dangerous = 'Bad html <script>alert(123);</script>';
+    $sanitized = Html::escape($dangerous);
+
+    $this->session
+      ->getResponseHeader('Content-type')
+      ->willReturn('text/html');
+    $this->webAssert
+      ->pageTextNotContains($dangerous)
+      ->shouldBeCalled();
+    $this->webAssert
+      ->pageTextNotContains($sanitized)
+      ->shouldNotBeCalled();
+
+    // The legacy ::assertNoText() returns nothing and calls no PHPUnit
+    // assertion, thus we need a real assertion. If ::assertText() fails, then
+    // it will return the exception and this test will fail.
+    $this->assertNull($this->assertNoText($sanitized));
+  }
+
+  /**
    * Returns a mocked behat session object.
    *
    * @return \Behat\Mink\Session
diff --git a/core/tests/Drupal/Tests/DocumentElement.php b/core/tests/Drupal/Tests/DocumentElement.php
new file mode 100644
index 0000000..323bde2
--- /dev/null
+++ b/core/tests/Drupal/Tests/DocumentElement.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\Tests;
+
+use DOMDocument;
+use Behat\Mink\Element\DocumentElement as MinkDocumentElement;
+
+/**
+ * Overrides the Mink document to better handle page text.
+ */
+class DocumentElement extends MinkDocumentElement {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getText() {
+    $raw_content = $this->getContent();
+    $dom = new DOMDocument();
+    // Suppress libxml warnings when loading HTML 5 content.
+    @$dom->loadHTML($raw_content);
+    // Collect DOM elements to be removed because DOMNodeList does not allow us
+    // to remove them directly when iterating over it.
+    $remove = [];
+
+    // Remove all JS script tags, they must never be part of the plain text
+    // content.
+    $scripts = $dom->getElementsByTagName('script');
+    foreach ($scripts as $script) {
+      $remove[] = $script;
+    }
+
+    // Remove all CSS style tags, they must never be part of the plain text
+    // content.
+    $styles = $dom->getElementsByTagName('style');
+    foreach ($styles as $style) {
+      $remove[] = $style;
+    }
+
+    foreach ($remove as $item) {
+      $item->parentNode->removeChild($item);
+    }
+
+    return $dom->textContent;
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Session.php b/core/tests/Drupal/Tests/Session.php
new file mode 100644
index 0000000..331fa68
--- /dev/null
+++ b/core/tests/Drupal/Tests/Session.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\Tests;
+
+use Behat\Mink\Session as MinkSession;
+use Behat\Mink\Driver\DriverInterface;
+use Behat\Mink\Selector\SelectorsHandler;
+
+/**
+ * Overrides the Mink session to better handle page text.
+ */
+class Session extends MinkSession {
+
+  /**
+   * The page document.
+   *
+   * The variable on the parent class is private, so we need to define it again
+   * here.
+   *
+   * @var \Drupal\Tests\DocumentElement
+   */
+  protected $page;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(DriverInterface $driver, SelectorsHandler $selectorsHandler = NULL) {
+      parent::__construct($driver, $selectorsHandler);
+
+      // Use our own document class.
+      $this->page = new DocumentElement($this);
+  }
+
+  /**
+   * {@inheirtdoc}
+   */
+  public function getPage() {
+    // The variable on the parent class is private, so we need to override this
+    // method.
+    return $this->page;
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/WebAssert.php b/core/tests/Drupal/Tests/WebAssert.php
index 6156dda..18b9e96 100644
--- a/core/tests/Drupal/Tests/WebAssert.php
+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -6,7 +6,6 @@
 use Behat\Mink\WebAssert as MinkWebAssert;
 use Behat\Mink\Element\TraversableElement;
 use Behat\Mink\Exception\ElementNotFoundException;
-use Behat\Mink\Session;
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Url;