diff --git a/lib/experimental/html/class-wp-html-tag-processor.php b/lib/experimental/html/class-wp-html-tag-processor.php
index 0dfa57f30f2aab..9539f0d626e9d6 100644
--- a/lib/experimental/html/class-wp-html-tag-processor.php
+++ b/lib/experimental/html/class-wp-html-tag-processor.php
@@ -1412,6 +1412,49 @@ public function get_attribute( $name ) {
 		return html_entity_decode( $raw_value );
 	}
 
+	/**
+	 * Returns the lowercase names of all attributes matching a given prefix in the currently-opened tag.
+	 *
+	 * Note that matching is case-insensitive. This is in accordance with the spec:
+	 *
+	 * > There must never be two or more attributes on
+	 * > the same start tag whose names are an ASCII
+	 * > case-insensitive match for each other.
+	 *     - HTML 5 spec
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/syntax.html#attributes-2:ascii-case-insensitive
+	 *
+	 * Example:
+	 * <code>
+	 *     $p = new WP_HTML_Tag_Processor( '<div data-ENABLED class="test" DATA-test-id="14">Test</div>' );
+	 *     $p->next_tag( [ 'class_name' => 'test' ] ) === true;
+	 *     $p->get_attribute_names_with_prefix( 'data-' ) === array( 'data-enabled', 'data-test-id' );
+	 *
+	 *     $p->next_tag( [] ) === false;
+	 *     $p->get_attribute_names_with_prefix( 'data-' ) === null;
+	 * </code>
+	 *
+	 * @since 6.2.0
+	 *
+	 * @param string $prefix Prefix of requested attribute names.
+	 * @return array|null List of attribute names, or `null` if not at a tag.
+	 */
+	function get_attribute_names_with_prefix( $prefix ) {
+		if ( $this->is_closing_tag || null === $this->tag_name_starts_at ) {
+			return null;
+		}
+
+		$comparable = strtolower( $prefix );
+
+		$matches = array();
+		foreach ( array_keys( $this->attributes ) as $attr_name ) {
+			if ( str_starts_with( $attr_name, $comparable ) ) {
+				$matches[] = $attr_name;
+			}
+		}
+		return $matches;
+	}
+
 	/**
 	 * Returns the lowercase name of the currently-opened tag.
 	 *
diff --git a/phpunit/html/wp-html-tag-processor-test.php b/phpunit/html/wp-html-tag-processor-test.php
index e6b3d4d8071af5..6750410f26d43c 100644
--- a/phpunit/html/wp-html-tag-processor-test.php
+++ b/phpunit/html/wp-html-tag-processor-test.php
@@ -74,6 +74,19 @@ public function test_get_attribute_returns_null_when_not_in_open_tag() {
 		$this->assertNull( $p->get_attribute( 'class' ), 'Accessing an attribute of a non-existing tag did not return null' );
 	}
 
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers next_tag
+	 * @covers get_attribute
+	 */
+	public function test_get_attribute_returns_null_when_in_closing_tag() {
+		$p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+		$this->assertTrue( $p->next_tag( 'div' ), 'Querying an existing tag did not return true' );
+		$this->assertTrue( $p->next_tag( array( 'tag_closers' => 'visit' ) ), 'Querying an existing closing tag did not return true' );
+		$this->assertNull( $p->get_attribute( 'class' ), 'Accessing an attribute of a closing tag did not return null' );
+	}
+
 	/**
 	 * @ticket 56299
 	 *
@@ -195,6 +208,87 @@ public function test_set_attribute_is_case_insensitive() {
 		$this->assertEquals( '<div data-enabled="abc">Test</div>', $p->get_updated_html(), 'A case-insensitive set_attribute call did not update the existing attribute.' );
 	}
 
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_null_before_finding_tags() {
+		$p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+		$this->assertNull( $p->get_attribute_names_with_prefix( 'data-' ) );
+	}
+
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_null_when_not_in_open_tag() {
+		$p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+		$p->next_tag( 'p' );
+		$this->assertNull( $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing attributes of a non-existing tag did not return null' );
+	}
+
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_null_when_in_closing_tag() {
+		$p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+		$p->next_tag( 'div' );
+		$p->next_tag( array( 'tag_closers' => 'visit' ) );
+		$this->assertNull( $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing attributes of a closing tag did not return null' );
+	}
+
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_empty_array_when_no_attributes_present() {
+		$p = new WP_HTML_Tag_Processor( '<div>Test</div>' );
+		$p->next_tag( 'div' );
+		$this->assertSame( array(), $p->get_attribute_names_with_prefix( 'data-' ), 'Accessing the attributes on a tag without any did not return an empty array' );
+	}
+
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_matching_attribute_names_in_lowercase() {
+		$p = new WP_HTML_Tag_Processor( '<div DATA-enabled class="test" data-test-ID="14">Test</div>' );
+		$p->next_tag();
+		$this->assertSame(
+			array( 'data-enabled', 'data-test-id' ),
+			$p->get_attribute_names_with_prefix( 'data-' )
+		);
+	}
+
+	/**
+	 * @ticket 56299
+	 *
+	 * @covers set_attribute
+	 * @covers get_updated_html
+	 * @covers get_attribute_names_with_prefix
+	 */
+	public function test_get_attribute_names_with_prefix_returns_attribute_added_by_set_attribute() {
+		$p = new WP_HTML_Tag_Processor( '<div data-foo="bar">Test</div>' );
+		$p->next_tag();
+		$p->set_attribute( 'data-test-id', '14' );
+		$this->assertSame(
+			'<div data-test-id="14" data-foo="bar">Test</div>',
+			$p->get_updated_html(),
+			"Updated HTML doesn't include attribute added via set_attribute"
+		);
+		$this->assertSame(
+			array( 'data-test-id', 'data-foo' ),
+			$p->get_attribute_names_with_prefix( 'data-' ),
+			"Accessing attribute names doesn't find attribute added via set_attribute"
+		);
+	}
+
 	/**
 	 * @ticket 56299
 	 *