diff --git a/lib/experimental/html/class-wp-html-tag-processor.php b/lib/experimental/html/class-wp-html-tag-processor.php
index 0d1c0756e843c..13fa6100e1d19 100644
--- a/lib/experimental/html/class-wp-html-tag-processor.php
+++ b/lib/experimental/html/class-wp-html-tag-processor.php
@@ -908,7 +908,9 @@ public function get_attribute( $name ) {
 			return true;
 		}
 
-		return substr( $this->html, $attribute->value_starts_at, $attribute->value_length );
+		$raw_value = substr( $this->html, $attribute->value_starts_at, $attribute->value_length );
+
+		return html_entity_decode( $raw_value );
 	}
 
 	/**
@@ -945,6 +947,8 @@ public function get_tag() {
 	 *  - When `true` is passed as the value, then only the attribute name is added to the tag.
 	 *  - When `false` is passed, the attribute gets removed if it existed before.
 	 *
+	 * For string attributes, the value is escaped using the `esc_attr` function.
+	 *
 	 * @since 6.2.0
 	 *
 	 * @param string         $name  The attribute name to target.
@@ -968,8 +972,7 @@ public function set_attribute( $name, $value ) {
 		if ( true === $value ) {
 			$updated_attribute = $name;
 		} else {
-			// @TODO: What escaping and sanitization do we need here?
-			$escaped_new_value = str_replace( '"', '"', $value );
+			$escaped_new_value = esc_attr( $value );
 			$updated_attribute = "{$name}=\"{$escaped_new_value}\"";
 		}
 
diff --git a/phpunit/html/wp-html-tag-processor-test.php b/phpunit/html/wp-html-tag-processor-standalone-test.php
similarity index 92%
rename from phpunit/html/wp-html-tag-processor-test.php
rename to phpunit/html/wp-html-tag-processor-standalone-test.php
index 41bf04a138abc..a09e982f7cb44 100644
--- a/phpunit/html/wp-html-tag-processor-test.php
+++ b/phpunit/html/wp-html-tag-processor-standalone-test.php
@@ -1,14 +1,23 @@
 <?php
 /**
  * Unit tests covering WP_HTML_Tag_Processor functionality.
+ * This file takes about 100ms to run because it does not load
+ * any WordPress libraries:
+ *
+ * ```
+ * ./vendor/bin/phpunit --no-configuration ./phpunit/html/wp-html-tag-processor-test.php
+ * ```
+ *
+ * Put all new WP_HTML_Tag_Processor tests here, and only add new cases to
+ * wp-html-tag-processor-test-wp.php when they cannot run without WordPress.
  *
  * @package WordPress
  * @subpackage HTML
  */
 
 if ( ! function_exists( 'esc_attr' ) ) {
-	function esc_attr( $s ) {
-		return str_replace( '"', '&quot;', $s );
+	function esc_attr( $string ) {
+		return htmlspecialchars( $string, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, 'utf-8', false );
 	}
 }
 
@@ -23,7 +32,7 @@ abstract class WP_UnitTestCase extends \PHPUnit\Framework\TestCase {}
  *
  * @coversDefaultClass WP_HTML_Tag_Processor
  */
-class WP_HTML_Tag_Processor_Test extends WP_UnitTestCase {
+class WP_HTML_Tag_Processor_Standalone_Test extends WP_UnitTestCase {
 	const HTML_SIMPLE       = '<div id="first"><span id="second">Text</span></div>';
 	const HTML_WITH_CLASSES = '<div class="main with-border" id="first"><span class="not-main bold with-border" id="second">Text</span></div>';
 	const HTML_MALFORMED    = '<div><span class="d-md-none" Notifications</span><span class="d-none d-md-inline">Back to notifications</span></div>';
@@ -134,6 +143,17 @@ public function test_get_attribute_returns_string_for_truthy_attributes() {
 		$this->assertSame( 'true', $p->get_attribute( 'hidden' ), 'Accessing a hidden="true" attribute value did not return "true"' );
 	}
 
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers WP_HTML_Tag_Processor::get_attribute
+	 */
+	public function test_get_attribute_decodes_html_character_references() {
+		$p = new WP_HTML_Tag_Processor( '<div id="the &quot;grande&quot; is &lt; &#x033;&#50;oz&dagger;"></div>' );
+		$p->next_tag();
+		$this->assertSame( 'the "grande" is < 32oz†', $p->get_attribute( 'id' ), 'HTML Attribute value was returned without decoding character references' );
+	}
+
 	/**
 	 * @ticket 56299
 	 *
@@ -235,6 +255,68 @@ public function test_set_attribute_on_a_non_existing_tag_does_not_change_the_mar
 		);
 	}
 
+	/**
+	 * Passing a double quote inside of an attribute values could lead to an XSS attack as follows:
+	 *
+	 * <code>
+	 *     $p = new WP_HTML_Tag_Processor( '<div class="header"></div>' );
+	 *     $p->next_tag();
+	 *     $p->set_attribute('class', '" onclick="alert');
+	 *     echo $p;
+	 *     // <div class="" onclick="alert"></div>
+	 * </code>
+	 *
+	 * To prevent it, `set_attribute` calls `esc_attr()` on its given values.
+	 *
+	 * <code>
+	 *    <div class="&quot; onclick=&quot;alert"></div>
+	 * </code>
+	 *
+	 * @ticket 56299
+	 *
+	 * @dataProvider data_set_attribute_escapable_values
+	 * @covers set_attribute
+	 */
+	public function test_set_attribute_prevents_xss( $attribute_value ) {
+		$p = new WP_HTML_Tag_Processor( '<div></div>' );
+		$p->next_tag();
+		$p->set_attribute( 'test', $attribute_value );
+
+		/*
+		 * Testing the escaping is hard using tools that properly parse
+		 * HTML because they might interpret the escaped values. It's hard
+		 * with tools that don't understand HTML because they might get
+		 * confused by improperly-escaped values.
+		 *
+		 * For this test, since we control the input HTML we're going to
+		 * do what looks like the opposite of what we want to be doing with
+		 * this library but are only doing so because we have full control
+		 * over the content and because we want to look at the raw values.
+		 */
+		$match = null;
+		preg_match( '~^<div test=(.*)></div>$~', (string) $p, $match );
+		list( , $actual_value ) = $match;
+
+		$this->assertEquals( $actual_value, '"' . esc_attr( $attribute_value ) . '"' );
+	}
+
+	/**
+	 * Data provider with HTML attribute values that might need escaping.
+	 */
+	public function data_set_attribute_escapable_values() {
+		return array(
+			array( '"' ),
+			array( '&quot;' ),
+			array( '&' ),
+			array( '&amp;' ),
+			array( '&euro;' ),
+			array( "'" ),
+			array( '<>' ),
+			array( '&quot";' ),
+			array( '" onclick="alert(\'1\');"><span onclick=""></span><script>alert("1")</script>' ),
+		);
+	}
+
 	/**
 	 * @ticket 56299
 	 *
@@ -999,12 +1081,12 @@ public function data_malformed_tag() {
 
 		$examples['HTML tag opening inside attribute value'] = array(
 			'<pre id="<code" class="wp-block-code <code is poetry&gt;"><code>This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
-			'<pre foo="bar" id="<code" class="wp-block-code <code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+			'<pre foo="bar" id="<code" class="wp-block-code &lt;code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
 		);
 
 		$examples['HTML tag brackets in attribute values and data markup'] = array(
 			'<pre id="<code-&gt;-block-&gt;" class="wp-block-code <code is poetry&gt;"><code>This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
-			'<pre foo="bar" id="<code-&gt;-block-&gt;" class="wp-block-code <code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
+			'<pre foo="bar" id="<code-&gt;-block-&gt;" class="wp-block-code &lt;code is poetry&gt; firstTag"><code class="secondTag">This &lt;is> a &lt;strong is="true">thing.</code></pre><span>test</span>',
 		);
 
 		$examples['Single and double quotes in attribute value'] = array(
diff --git a/phpunit/html/wp-html-tag-processor-wp-test.php b/phpunit/html/wp-html-tag-processor-wp-test.php
new file mode 100644
index 0000000000000..6c3a30340b267
--- /dev/null
+++ b/phpunit/html/wp-html-tag-processor-wp-test.php
@@ -0,0 +1,91 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Tag_Processor functionality.
+ * This file is only for tests that require WordPress libraries.
+ * Put all other tests in wp-html-tag-processor-test-standalone.php.
+ *
+ * Run these tests locally as follows:
+ *
+ * ```
+ * npx wp-env run phpunit "phpunit -c /var/www/html/wp-content/plugins/gutenberg/phpunit.xml.dist --filter WP_HTML_Tag_Processor_Test_WP"
+ * ```
+ *
+ * @package WordPress
+ * @subpackage HTML
+ */
+
+require_once __DIR__ . '/../../lib/experimental/html/index.php';
+
+/**
+ * @group html
+ *
+ * @coversDefaultClass WP_HTML_Tag_Processor
+ */
+class WP_HTML_Tag_Processor_Test_WP extends WP_UnitTestCase {
+
+	/**
+	 * Passing a double quote inside of an attribute values could lead to an XSS attack as follows:
+	 *
+	 * <code>
+	 *     $p = new WP_HTML_Tag_Processor( '<div class="header"></div>' );
+	 *     $p->next_tag();
+	 *     $p->set_attribute('class', '" onclick="alert');
+	 *     echo $p;
+	 *     // <div class="" onclick="alert"></div>
+	 * </code>
+	 *
+	 * To prevent it, `set_attribute` calls `esc_attr()` on its given values.
+	 *
+	 * <code>
+	 *    <div class="&quot; onclick=&quot;alert"></div>
+	 * </code>
+	 *
+	 * @ticket 56299
+	 *
+	 * @dataProvider data_set_attribute_escapable_values
+	 * @covers set_attribute
+	 */
+	public function test_set_attribute_prevents_xss( $value_to_set, $expected_result ) {
+		$p = new WP_HTML_Tag_Processor( '<div></div>' );
+		$p->next_tag();
+		$p->set_attribute( 'test', $value_to_set );
+
+		/*
+		 * Testing the escaping is hard using tools that properly parse
+		 * HTML because they might interpret the escaped values. It's hard
+		 * with tools that don't understand HTML because they might get
+		 * confused by improperly-escaped values.
+		 *
+		 * For this test, since we control the input HTML we're going to
+		 * do what looks like the opposite of what we want to be doing with
+		 * this library but are only doing so because we have full control
+		 * over the content and because we want to look at the raw values.
+		 */
+		$match = null;
+		preg_match( '~^<div test=(.*)></div>$~', (string) $p, $match );
+		list( , $actual_value ) = $match;
+
+		$this->assertEquals( $actual_value, '"' . $expected_result . '"' );
+	}
+
+	/**
+	 * Data provider with HTML attribute values that might need escaping.
+	 */
+	public function data_set_attribute_escapable_values() {
+		return array(
+			array( '"', '&quot;' ),
+			array( '&quot;', '&quot;' ),
+			array( '&', '&amp;' ),
+			array( '&amp;', '&amp;' ),
+			array( '&euro;', '&euro;' ),
+			array( "'", '&#039;' ),
+			array( '<>', '&lt;&gt;' ),
+			array( '&quot";', '&amp;quot&quot;;' ),
+			array(
+				'" onclick="alert(\'1\');"><span onclick=""></span><script>alert("1")</script>',
+				'&quot; onclick=&quot;alert(&#039;1&#039;);&quot;&gt;&lt;span onclick=&quot;&quot;&gt;&lt;/span&gt;&lt;script&gt;alert(&quot;1&quot;)&lt;/script&gt;',
+			),
+		);
+	}
+
+}