diff --git a/wp-includes/html-api/class-wp-html-processor.php b/wp-includes/html-api/class-wp-html-processor.php
index 64fd23924e..52735f29e6 100644
--- a/wp-includes/html-api/class-wp-html-processor.php
+++ b/wp-includes/html-api/class-wp-html-processor.php
@@ -27,6 +27,7 @@
  *  - Unwrapping a tag by removing its parent.
  *  - Inserting and removing nodes.
  *  - Reading and changing inner content.
+ *  - Navigating up or around HTML structure.
  *
  * ## Usage
  *
@@ -66,7 +67,7 @@
  * `array( 'IMG' )` matches all IMG elements and `array( 'P', 'IMG' )`
  * matches all IMG elements directly inside a P element. To ensure that no
  * partial matches erroneously match it's possible to specify in a query
- * the full breadcrumb match.
+ * the full breadcrumb match all the way down from the root HTML element.
  *
  * Example:
  *
@@ -76,7 +77,7 @@
  *
  *     $html = '<figure><img><figcaption>A <em>lovely</em> day outside</figcaption></figure>';
  *     //                                  ---- Matches here.
- *     $processor->next_tag( array( 'breadcrumbs' => array( 'FIGURE', 'IMG', 'FIGCAPTION', 'EM' ) ) );
+ *     $processor->next_tag( array( 'breadcrumbs' => array( 'FIGURE', 'FIGCAPTION', 'EM' ) ) );
  *
  *     $html = '<div><img></div><img>';
  *     //                       ----- Matches here, because IMG must be a direct child of the implicit BODY.
@@ -100,7 +101,8 @@
  *
  *  - Links: A.
  *  - The formatting elements: B, BIG, CODE, EM, FONT, I, SMALL, STRIKE, STRONG, TT, U.
- *  - Containers: DIV, FIGCAPTION, FIGURE.
+ *  - Containers: DIV, FIGCAPTION, FIGURE, SPAN.
+ *  - Form elements: BUTTON.
  *  - Paragraph: P.
  *  - Void elements: IMG.
  *
@@ -116,7 +118,9 @@
  *  - Markup involving only those tags listed above.
  *  - Fully-balanced and non-overlapping tags.
  *  - HTML with unexpected tag closers.
+ *  - Some unbalanced or overlapping tags.
  *  - P tags after unclosed P tags.
+ *  - BUTTON tags after unclosed BUTTON tags.
  *  - A tags after unclosed A tags that don't involve any active formatting elements.
  *
  * @since 6.4.0
@@ -126,7 +130,10 @@
  */
 class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
-	 * HTML processing requires more bookmarks than basic tag processing.
+	 * The maximum number of bookmarks allowed to exist at any given time.
+	 *
+	 * HTML processing requires more bookmarks than basic tag processing,
+	 * so this class constant from the Tag Processor is overwritten.
 	 *
 	 * @since 6.4.0
 	 *
@@ -137,6 +144,8 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
 	 * Static query for instructing the Tag Processor to visit every token.
 	 *
+	 * @access private
+	 *
 	 * @since 6.4.0
 	 *
 	 * @var array
@@ -158,8 +167,15 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
 	 * Used to create unique bookmark names.
 	 *
+	 * This class sets a bookmark for every tag in the HTML document that it encounters.
+	 * The bookmark name is auto-generated and increments, starting with `1`. These are
+	 * internal bookmarks and are automatically released when the referring WP_HTML_Token
+	 * goes out of scope and is garbage-collected.
+	 *
 	 * @since 6.4.0
 	 *
+	 * @see WP_HTML_Processor::$release_internal_bookmark_on_destruct
+	 *
 	 * @var int
 	 */
 	private $bookmark_counter = 0;
@@ -186,7 +202,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 *
 	 * @var closure
 	 */
-	private $release_internal_bookmark = null;
+	private $release_internal_bookmark_on_destruct = null;
 
 	/*
 	 * Public Interface Functions
@@ -258,10 +274,14 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
 	 * Constructor.
 	 *
+	 * Do not use this method. Use the static creator methods instead.
+	 *
 	 * @access private
 	 *
 	 * @since 6.4.0
 	 *
+	 * @see WP_HTML_Processor::createFragment()
+	 *
 	 * @param string      $html                                  HTML to process.
 	 * @param string|null $use_the_static_create_methods_instead This constructor should not be called manually.
 	 */
@@ -287,7 +307,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 		 * a private method into WP_HTML_Token classes without
 		 * exposing it to any public API.
 		 */
-		$this->release_internal_bookmark = function ( $name ) {
+		$this->release_internal_bookmark_on_destruct = function ( $name ) {
 			parent::release_bookmark( $name );
 		};
 	}
@@ -298,13 +318,15 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 * Various situations lead to parsing failure but this class will
 	 * return `false` in all those cases. To determine why something
 	 * failed it's possible to request the last error. This can be
-	 * helpful to know if it's possible to fix something or to give up.
+	 * helpful to know to distinguish whether a given tag couldn't
+	 * be found or if content in the document caused the processor
+	 * to give up and abort processing.
 	 *
 	 * Example
 	 *
-	 *     $p = WP_HTML_Processor::createFragment( '<template><strong><button><em><p><em>' );
-	 *     false === $p->next_tag();
-	 *     WP_HTML_Processor::ERROR_UNSUPPORTED === $p->get_last_error();
+	 *     $processor = WP_HTML_Processor::createFragment( '<template><strong><button><em><p><em>' );
+	 *     false === $processor->next_tag();
+	 *     WP_HTML_Processor::ERROR_UNSUPPORTED === $processor->get_last_error();
 	 *
 	 * @since 6.4.0
 	 *
@@ -429,6 +451,20 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 		}
 
 		if ( self::PROCESS_NEXT_NODE === $node_to_process ) {
+			/*
+			 * Void elements still hop onto the stack of open elements even though
+			 * there's no corresponding closing tag. This is important for managing
+			 * stack-based operations such as "navigate to parent node" or checking
+			 * on an element's breadcrumbs.
+			 *
+			 * When moving on to the next node, therefore, if the bottom-most element
+			 * on the stack is a void element, it must be closed.
+			 *
+			 * @TODO: Once self-closing foreign elements and BGSOUND are supported,
+			 *        they must also be implicitly closed here too. BGSOUND is
+			 *        special since it's only self-closing if the self-closing flag
+			 *        is provided in the opening tag, otherwise it expects a tag closer.
+			 */
 			$top_node = $this->state->stack_of_open_elements->current_node();
 			if ( $top_node && self::is_void( $top_node->node_name ) ) {
 				$this->state->stack_of_open_elements->pop();
@@ -446,7 +482,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 			$this->bookmark_tag(),
 			$this->get_tag(),
 			$this->is_tag_closer(),
-			$this->release_internal_bookmark
+			$this->release_internal_bookmark_on_destruct
 		);
 
 		try {
@@ -456,7 +492,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 
 				default:
 					$this->last_error = self::ERROR_UNSUPPORTED;
-					throw new WP_HTML_Unsupported_Exception( 'Cannot parse outside of the IN BODY insertion mode.' );
+					throw new WP_HTML_Unsupported_Exception( "No support for parsing in the '{$this->state->insertion_mode}' state." );
 			}
 		} catch ( WP_HTML_Unsupported_Exception $e ) {
 			/*
@@ -470,17 +506,22 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
 	 * Computes the HTML breadcrumbs for the currently-matched node, if matched.
 	 *
-	 * Breadcrumbs start at the outer-most parent and descend toward the matched element.
+	 * Breadcrumbs start at the outermost parent and descend toward the matched element.
+	 * They always include the entire path from the root HTML node to the matched element.
+	 *
+	 * @TODO: It could be more efficient to expose a generator-based version of this function
+	 *        to avoid creating the array copy on tag iteration. If this is done, it would likely
+	 *        be more useful to walk up the stack when yielding instead of starting at the top.
 	 *
 	 * Example
 	 *
-	 *     $p = WP_HTML_Processor::createFragment( '<p><strong><em><img></em></strong></p>' );
-	 *     $p->next_tag( 'IMG' );
-	 *     $p->get_breadcrumbs() === array( 'HTML', 'BODY', 'P', 'STRONG', 'EM', 'IMG' );
+	 *     $processor = WP_HTML_Processor::createFragment( '<p><strong><em><img></em></strong></p>' );
+	 *     $processor->next_tag( 'IMG' );
+	 *     $processor->get_breadcrumbs() === array( 'HTML', 'BODY', 'P', 'STRONG', 'EM', 'IMG' );
 	 *
 	 * @since 6.4.0
 	 *
-	 * @return string[]|null Array of tag-names representing path to matched node, if matched, otherwise null.
+	 * @return string[]|null Array of tag names representing path to matched node, if matched, otherwise NULL.
 	 */
 	public function get_breadcrumbs() {
 		if ( ! $this->get_tag() ) {
@@ -499,16 +540,14 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 * Parses next element in the 'in body' insertion mode.
 	 *
 	 * This internal function performs the 'in body' insertion mode
-	 * logic for the generalized `self::step()` function.
-	 *
-	 * @access private
+	 * logic for the generalized WP_HTML_Processor::step() function.
 	 *
 	 * @since 6.4.0
 	 *
 	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
 	 *
 	 * @see https://html.spec.whatwg.org/#parsing-main-inbody
-	 * @see self::step
+	 * @see WP_HTML_Processor::step
 	 *
 	 * @return bool Whether an element was found.
 	 */
@@ -698,7 +737,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 */
 
 	/**
-	 * Creates a new bookmark for the currently-matched tag and returns generated name.
+	 * Creates a new bookmark for the currently-matched tag and returns the generated name.
 	 *
 	 * @since 6.4.0
 	 *
@@ -726,14 +765,19 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	/**
 	 * Returns the uppercase name of the matched tag.
 	 *
+	 * The semantic rules for HTML specify that certain tags be reprocessed
+	 * with a different tag name. Because of this, the tag name presented
+	 * by the HTML Processor may differ from the one reported by the HTML
+	 * Tag Processor, which doesn't apply these semantic rules.
+	 *
 	 * Example:
 	 *
-	 *     $p = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
-	 *     $p->next_tag() === true;
-	 *     $p->get_tag() === 'DIV';
+	 *     $processor = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
+	 *     $processor->next_tag() === true;
+	 *     $processor->get_tag() === 'DIV';
 	 *
-	 *     $p->next_tag() === false;
-	 *     $p->get_tag() === null;
+	 *     $processor->next_tag() === false;
+	 *     $processor->get_tag() === null;
 	 *
 	 * @since 6.4.0
 	 *
@@ -775,11 +819,13 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	}
 
 	/**
-	 * Moves the internal cursor in the Tag Processor to a given bookmark's location.
+	 * Moves the internal cursor in the HTML Processor to a given bookmark's location.
 	 *
 	 * In order to prevent accidental infinite loops, there's a
 	 * maximum limit on the number of times seek() can be called.
 	 *
+	 * @throws Exception When unable to allocate a bookmark for the next token in the input HTML document.
+	 *
 	 * @since 6.4.0
 	 *
 	 * @param string $bookmark_name Jump to the place in the document identified by this bookmark name.
@@ -1348,6 +1394,8 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 *
 	 * This unlock code is used to ensure that anyone calling the constructor is
 	 * doing so with a full understanding that it's intended to be a private API.
+	 *
+	 * @access private
 	 */
 	const CONSTRUCTOR_UNLOCK_CODE = 'Use WP_HTML_Processor::createFragment instead of calling the class constructor directly.';
 }
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 934c6dc88c..4fba75fd1c 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56563';
+$wp_version = '6.4-alpha-56565';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.