diff --git a/src/wp-includes/html-api/class-wp-html-open-elements.php b/src/wp-includes/html-api/class-wp-html-open-elements.php
index b1ca2a5dfa918..d1585cdea5bf5 100644
--- a/src/wp-includes/html-api/class-wp-html-open-elements.php
+++ b/src/wp-includes/html-api/class-wp-html-open-elements.php
@@ -189,11 +189,6 @@ public function current_node_is( string $identity ): bool {
 	/**
 	 * Returns whether an element is in a specific scope.
 	 *
-	 * ## HTML Support
-	 *
-	 * This function skips checking for the termination list because there
-	 * are no supported elements which appear in the termination list.
-	 *
 	 * @since 6.4.0
 	 *
 	 * @see https://html.spec.whatwg.org/#has-an-element-in-the-specific-scope
@@ -311,19 +306,38 @@ public function has_element_in_table_scope( $tag_name ) {
 	/**
 	 * Returns whether a particular element is in select scope.
 	 *
-	 * @since 6.4.0
+	 * This test differs from the others like it, in that its rules are inverted.
+	 * Instead of arriving at a match when one of any tag in a termination group
+	 * is reached, this one terminates if any other tag is reached.
 	 *
-	 * @see https://html.spec.whatwg.org/#has-an-element-in-select-scope
+	 * > The stack of open elements is said to have a particular element in select scope when it has
+	 * > that element in the specific scope consisting of all element types except the following:
+	 * >   - optgroup in the HTML namespace
+	 * >   - option in the HTML namespace
 	 *
-	 * @throws WP_HTML_Unsupported_Exception Always until this function is implemented.
+	 * @since 6.4.0 Stub implementation (throws).
+	 * @since 6.7.0 Full implementation.
+	 *
+	 * @see https://html.spec.whatwg.org/#has-an-element-in-select-scope
 	 *
 	 * @param string $tag_name Name of tag to check.
-	 * @return bool Whether given element is in scope.
+	 * @return bool Whether the given element is in SELECT scope.
 	 */
 	public function has_element_in_select_scope( $tag_name ) {
-		throw new WP_HTML_Unsupported_Exception( 'Cannot process elements depending on select scope.' );
+		foreach ( $this->walk_up() as $node ) {
+			if ( $node->node_name === $tag_name ) {
+				return true;
+			}
 
-		return false; // The linter requires this unreachable code until the function is implemented and can return.
+			if (
+				'OPTION' !== $node->node_name &&
+				'OPTGROUP' !== $node->node_name
+			) {
+				return false;
+			}
+		}
+
+		return false;
 	}
 
 	/**
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 f187ef7fd1b66..40538491152ad 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -101,7 +101,7 @@
  *
  *  - Containers: ADDRESS, BLOCKQUOTE, DETAILS, DIALOG, DIV, FOOTER, HEADER, MAIN, MENU, SPAN, SUMMARY.
  *  - Custom elements: All custom elements are supported. :)
- *  - Form elements: BUTTON, DATALIST, FIELDSET, INPUT, LABEL, LEGEND, METER, PROGRESS, SEARCH.
+ *  - Form elements: BUTTON, DATALIST, FIELDSET, INPUT, LABEL, LEGEND, METER, OPTGROUP, OPTION, PROGRESS, SEARCH, SELECT.
  *  - Formatting elements: B, BIG, CODE, EM, FONT, I, PRE, SMALL, STRIKE, STRONG, TT, U, WBR.
  *  - Heading elements: H1, H2, H3, H4, H5, H6, HGROUP.
  *  - Links: A.
@@ -757,6 +757,12 @@ public function step( $node_to_process = self::PROCESS_NEXT_NODE ) {
 				case WP_HTML_Processor_State::INSERTION_MODE_IN_BODY:
 					return $this->step_in_body();
 
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD:
+					return $this->step_in_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT:
+					return $this->step_in_select();
+
 				default:
 					$this->last_error = self::ERROR_UNSUPPORTED;
 					throw new WP_HTML_Unsupported_Exception( "No support for parsing in the '{$this->state->insertion_mode}' state." );
@@ -1336,6 +1342,48 @@ private function step_in_body() {
 			case '+TRACK':
 				$this->insert_html_element( $this->state->current_token );
 				return true;
+
+			/*
+			 * > A start tag whose tag name is "select"
+			 */
+			case '+SELECT':
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok = false;
+
+				switch ( $this->state->insertion_mode ) {
+					/*
+					 * > If the insertion mode is one of "in table", "in caption", "in table body", "in row",
+					 * > or "in cell", then switch the insertion mode to "in select in table".
+					 */
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_ROW:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_CELL:
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE;
+						break;
+
+					/*
+					 * > Otherwise, switch the insertion mode to "in select".
+					 */
+					default:
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT;
+						break;
+				}
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "optgroup", "option"
+			 */
+			case '+OPTGROUP':
+			case '+OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				return true;
 		}
 
 		/*
@@ -1378,8 +1426,6 @@ private function step_in_body() {
 			case 'NOFRAMES':
 			case 'NOSCRIPT':
 			case 'OBJECT':
-			case 'OPTGROUP':
-			case 'OPTION':
 			case 'PLAINTEXT':
 			case 'RB':
 			case 'RP':
@@ -1387,7 +1433,6 @@ private function step_in_body() {
 			case 'RTC':
 			case 'SARCASM':
 			case 'SCRIPT':
-			case 'SELECT':
 			case 'STYLE':
 			case 'SVG':
 			case 'TABLE':
@@ -1448,6 +1493,207 @@ private function step_in_body() {
 		}
 	}
 
+	/**
+	 * Parses next element in the 'in head' insertion mode.
+	 *
+	 * This internal function performs the 'in head' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_head() {
+		$this->last_error = self::ERROR_UNSUPPORTED;
+		throw new WP_HTML_Unsupported_Exception( "No support for parsing in the '{$this->state->insertion_mode}' state." );
+	}
+
+	/**
+	 * Parses next element in the 'in select' insertion mode.
+	 *
+	 * This internal function performs the 'in select' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inselect
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_select() {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > Any other character token
+			 */
+			case '#text':
+				$current_token = $this->bookmarks[ $this->state->current_token->bookmark_name ];
+
+				/*
+				 * > A character token that is U+0000 NULL
+				 *
+				 * If a text node only comprises null bytes then it should be
+				 * entirely ignored and should not return to calling code.
+				 */
+				if (
+					1 <= $current_token->length &&
+					"\x00" === $this->html[ $current_token->start ] &&
+					strspn( $this->html, "\x00", $current_token->start, $current_token->length ) === $current_token->length
+				) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "option"
+			 */
+			case '+OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "optgroup"
+			 * > A start tag whose tag name is "hr"
+			 *
+			 * These rules are identical except for the treatment of the self-closing flag and
+			 * the subsequent pop of the HR void element, all of which is handled elsewhere in the processor.
+			 */
+			case '+OPTGROUP':
+			case '+HR':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTGROUP' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "optgroup"
+			 */
+			case '-OPTGROUP':
+				$current_node = $this->state->stack_of_open_elements->current_node();
+				if ( $current_node && 'OPTION' === $current_node->node_name ) {
+					foreach ( $this->state->stack_of_open_elements->walk_up( $current_node ) as $parent ) {
+						break;
+					}
+					if ( $parent && 'OPTGROUP' === $parent->node_name ) {
+						$this->state->stack_of_open_elements->pop();
+					}
+				}
+
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTGROUP' ) ) {
+					$this->state->stack_of_open_elements->pop();
+					return true;
+				}
+
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > An end tag whose tag name is "option"
+			 */
+			case '-OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+					return true;
+				}
+
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > An end tag whose tag name is "select"
+			 * > A start tag whose tag name is "select"
+			 *
+			 * > It just gets treated like an end tag.
+			 */
+			case '-SELECT':
+			case '+SELECT':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_select_scope( 'SELECT' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode();
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "input", "keygen", "textarea"
+			 *
+			 * All three of these tags are considered a parse error when found in this insertion mode.
+			 */
+			case '+INPUT':
+			case '+KEYGEN':
+			case '+TEXTAREA':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_select_scope( 'SELECT' ) ) {
+					// Ignore the token.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is one of: "script", "template"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+SCRIPT':
+			case '+TEMPLATE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+		}
+
+		/*
+		 * > Anything else
+		 * >   Parse error: ignore the token.
+		 */
+		return $this->step();
+	}
+
 	/*
 	 * Internal helpers
 	 */
@@ -2036,6 +2282,7 @@ private function close_a_p_element() {
 	 * Closes elements that have implied end tags.
 	 *
 	 * @since 6.4.0
+	 * @since 6.7.0 Full spec support.
 	 *
 	 * @see https://html.spec.whatwg.org/#generate-implied-end-tags
 	 *
@@ -2046,12 +2293,19 @@ private function generate_implied_end_tags( $except_for_this_element = null ) {
 			'DD',
 			'DT',
 			'LI',
+			'OPTGROUP',
+			'OPTION',
 			'P',
+			'RB',
+			'RP',
+			'RT',
+			'RTC',
 		);
 
-		$current_node = $this->state->stack_of_open_elements->current_node();
+		$no_exclusions = ! isset( $except_for_this_element );
+
 		while (
-			$current_node && $current_node->node_name !== $except_for_this_element &&
+			( $no_exclusions || ! $this->state->stack_of_open_elements->current_node_is( $except_for_this_element ) ) &&
 			in_array( $this->state->stack_of_open_elements->current_node(), $elements_with_implied_end_tags, true )
 		) {
 			$this->state->stack_of_open_elements->pop();
@@ -2065,16 +2319,31 @@ private function generate_implied_end_tags( $except_for_this_element = null ) {
 	 * different from generating end tags in the normal sense.
 	 *
 	 * @since 6.4.0
+	 * @since 6.7.0 Full spec support.
 	 *
 	 * @see WP_HTML_Processor::generate_implied_end_tags
 	 * @see https://html.spec.whatwg.org/#generate-implied-end-tags
 	 */
 	private function generate_implied_end_tags_thoroughly() {
 		$elements_with_implied_end_tags = array(
+			'CAPTION',
+			'COLGROUP',
 			'DD',
 			'DT',
 			'LI',
+			'OPTGROUP',
+			'OPTION',
 			'P',
+			'RB',
+			'RP',
+			'RT',
+			'RTC',
+			'TBODY',
+			'TD',
+			'TFOOT',
+			'TH',
+			'THEAD',
+			'TR',
 		);
 
 		while ( in_array( $this->state->stack_of_open_elements->current_node(), $elements_with_implied_end_tags, true ) ) {
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessor.php b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
index 536f6fdf4dd8f..b842703a7a135 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
@@ -406,8 +406,6 @@ public static function data_unsupported_special_in_body_tags() {
 			'NOFRAMES'  => array( 'NOFRAMES' ),
 			'NOSCRIPT'  => array( 'NOSCRIPT' ),
 			'OBJECT'    => array( 'OBJECT' ),
-			'OPTGROUP'  => array( 'OPTGROUP' ),
-			'OPTION'    => array( 'OPTION' ),
 			'PLAINTEXT' => array( 'PLAINTEXT' ),
 			'RB'        => array( 'RB' ),
 			'RP'        => array( 'RP' ),
@@ -415,7 +413,6 @@ public static function data_unsupported_special_in_body_tags() {
 			'RTC'       => array( 'RTC' ),
 			'SARCASM'   => array( 'SARCASM' ),
 			'SCRIPT'    => array( 'SCRIPT' ),
-			'SELECT'    => array( 'SELECT' ),
 			'STYLE'     => array( 'STYLE' ),
 			'SVG'       => array( 'SVG' ),
 			'TABLE'     => array( 'TABLE' ),
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessorBreadcrumbs.php b/tests/phpunit/tests/html-api/wpHtmlProcessorBreadcrumbs.php
index 01dcd9c32d7fb..403f40a1da032 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessorBreadcrumbs.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessorBreadcrumbs.php
@@ -189,15 +189,12 @@ public static function data_unsupported_elements() {
 			'NOFRAMES', // Neutralized.
 			'NOSCRIPT',
 			'OBJECT',
-			'OPTGROUP',
-			'OPTION',
 			'PLAINTEXT', // Neutralized.
 			'RB', // Neutralized.
 			'RP',
 			'RT',
 			'RTC', // Neutralized.
 			'SCRIPT',
-			'SELECT',
 			'STYLE',
 			'SVG',
 			'TABLE',
diff --git a/tests/phpunit/tests/html-api/wpHtmlSupportRequiredHtmlProcessor.php b/tests/phpunit/tests/html-api/wpHtmlSupportRequiredHtmlProcessor.php
index 2d3cd21ce461b..07943cd62a2f4 100644
--- a/tests/phpunit/tests/html-api/wpHtmlSupportRequiredHtmlProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlSupportRequiredHtmlProcessor.php
@@ -58,8 +58,6 @@ private function ensure_support_is_added_everywhere( $tag_name ) {
 	 * @covers WP_HTML_Processor::generate_implied_end_tags
 	 */
 	public function test_generate_implied_end_tags_needs_support() {
-		$this->ensure_support_is_added_everywhere( 'OPTGROUP' );
-		$this->ensure_support_is_added_everywhere( 'OPTION' );
 		$this->ensure_support_is_added_everywhere( 'RB' );
 		$this->ensure_support_is_added_everywhere( 'RP' );
 		$this->ensure_support_is_added_everywhere( 'RT' );
@@ -79,8 +77,6 @@ public function test_generate_implied_end_tags_needs_support() {
 	public function test_generate_implied_end_tags_thoroughly_needs_support() {
 		$this->ensure_support_is_added_everywhere( 'CAPTION' );
 		$this->ensure_support_is_added_everywhere( 'COLGROUP' );
-		$this->ensure_support_is_added_everywhere( 'OPTGROUP' );
-		$this->ensure_support_is_added_everywhere( 'OPTION' );
 		$this->ensure_support_is_added_everywhere( 'RB' );
 		$this->ensure_support_is_added_everywhere( 'RP' );
 		$this->ensure_support_is_added_everywhere( 'RT' );
diff --git a/tests/phpunit/tests/html-api/wpHtmlSupportRequiredOpenElements.php b/tests/phpunit/tests/html-api/wpHtmlSupportRequiredOpenElements.php
index c2e8c697e8156..48255190ad50c 100644
--- a/tests/phpunit/tests/html-api/wpHtmlSupportRequiredOpenElements.php
+++ b/tests/phpunit/tests/html-api/wpHtmlSupportRequiredOpenElements.php
@@ -308,9 +308,5 @@ public function test_has_element_in_select_scope_needs_support() {
 		 * FOREIGNOBJECT, DESC, TITLE.
 		 */
 		$this->ensure_support_is_added_everywhere( 'SVG' );
-
-		// These elements are specific to SELECT scope.
-		$this->ensure_support_is_added_everywhere( 'OPTGROUP' );
-		$this->ensure_support_is_added_everywhere( 'OPTION' );
 	}
 }