diff --git a/src/wp-includes/html-api/class-wp-html-processor-state.php b/src/wp-includes/html-api/class-wp-html-processor-state.php
index 9cf10c344107a..f72b6ebd765c9 100644
--- a/src/wp-includes/html-api/class-wp-html-processor-state.php
+++ b/src/wp-includes/html-api/class-wp-html-processor-state.php
@@ -59,6 +59,114 @@ class WP_HTML_Processor_State {
 	 */
 	const INSERTION_MODE_IN_BODY = 'insertion-mode-in-body';
 
+	/**
+	 * In select insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inselect
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_SELECT = 'insertion-mode-in-select';
+
+	/**
+	 * In select in table insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inselectintable
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_SELECT_IN_TABLE = 'insertion-mode-in-select-in-table';
+
+	/**
+	 * In table insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intable
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_TABLE = 'insertion-mode-in-table';
+
+	/**
+	 * In caption insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-incaption
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_CAPTION = 'insertion-mode-in-caption';
+
+	/**
+	 * In table body insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intablebody
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_TABLE_BODY = 'insertion-mode-in-table-body';
+
+	/**
+	 * In row insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inrow
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_ROW = 'insertion-mode-in-row';
+
+	/**
+	 * In cell insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-incell
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_CELL = 'insertion-mode-in-cell';
+
+	/**
+	 * In column group insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-incolumngroup
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_COLUMN_GROUP = 'insertion-mode-in-column-group';
+
+	/**
+	 * In frameset insertion mode for full HTML parser.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inframeset
+	 * @see WP_HTML_Processor_State::$insertion_mode
+	 *
+	 * @var string
+	 */
+	const INSERTION_MODE_IN_FRAMESET = 'insertion-mode-in-frameset';
+
 	/**
 	 * Tracks open elements while scanning HTML.
 	 *
diff --git a/src/wp-includes/html-api/class-wp-html-processor.php b/src/wp-includes/html-api/class-wp-html-processor.php
index 29f1c7ac6d4cc..d597be34d12fa 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -2129,6 +2129,86 @@ private function reconstruct_active_formatting_elements() {
 		throw new WP_HTML_Unsupported_Exception( 'Cannot reconstruct active formatting elements when advancing and rewinding is required.' );
 	}
 
+	/**
+	 * Runs the reset the insertion mode appropriately algorithm.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encoutering unsupported nodes.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#reset-the-insertion-mode-appropriately
+	 */
+	private function reset_insertion_mode() {
+		foreach ( $this->state->stack_of_open_elements->walk_up() as $node ) {
+			switch ( $node->node_name ) {
+				case 'SELECT':
+					foreach ( $this->state->stack_of_open_elements->walk_up( $node ) as $ancestor ) {
+						switch ( $ancestor ) {
+							// > If ancestor is a table node, switch the insertion mode to "in select in table" and return.
+							case 'TABLE':
+								$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE;
+								return;
+							// > If ancestor is a template node, jump to the step below labeled done.
+							case 'TEMPLATE':
+								break 2;
+						}
+					}
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT;
+					return;
+
+				case 'TD':
+				case 'TH':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CELL;
+					return;
+
+				case 'TR':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+					return;
+
+				case 'TBODY':
+				case 'THEAD':
+				case 'TFOOT':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+					return;
+
+				case 'CAPTION':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION;
+					return;
+
+				case 'COLGROUP':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+					return;
+
+				case 'TABLE':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+					return;
+
+				case 'TEMPLATE':
+					$this->last_error = self::ERROR_UNSUPPORTED;
+					throw new WP_HTML_Unsupported_Exception( 'Cannot reset insertion mode at TEMPLATE node.' );
+
+				case 'HEAD':
+					$this->last_error = self::ERROR_UNSUPPORTED;
+					throw new WP_HTML_Unsupported_Exception( 'Cannot reset insertion mode at HEAD node.' );
+
+				case 'BODY':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+					return;
+
+				case 'FRAMESET':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_FRAMESET;
+					return;
+
+				case 'HTML':
+					$this->last_error = self::ERROR_UNSUPPORTED;
+					throw new WP_HTML_Unsupported_Exception( 'Cannot reset insertion mode at HTML node.' );
+			}
+		}
+
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+		return;
+	}
+
 	/**
 	 * Runs the adoption agency algorithm.
 	 *