diff --git a/wp-includes/html-api/class-wp-html-open-elements.php b/wp-includes/html-api/class-wp-html-open-elements.php
index 1162a267f9..c760009ce0 100644
--- a/wp-includes/html-api/class-wp-html-open-elements.php
+++ b/wp-includes/html-api/class-wp-html-open-elements.php
@@ -308,7 +308,20 @@ class WP_HTML_Open_Elements {
 				'MARQUEE',
 				'OBJECT',
 				'TEMPLATE',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}
@@ -349,7 +362,20 @@ class WP_HTML_Open_Elements {
 				'OL',
 				'TEMPLATE',
 				'UL',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}
@@ -386,7 +412,20 @@ class WP_HTML_Open_Elements {
 				'MARQUEE',
 				'OBJECT',
 				'TEMPLATE',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}
diff --git a/wp-includes/html-api/class-wp-html-processor.php b/wp-includes/html-api/class-wp-html-processor.php
index f865302245..9f2662c9e4 100644
--- a/wp-includes/html-api/class-wp-html-processor.php
+++ b/wp-includes/html-api/class-wp-html-processor.php
@@ -1040,7 +1040,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 * This internal function performs the 'in head' insertion mode
 	 * logic for the generalized WP_HTML_Processor::step() function.
 	 *
-	 * @since 6.7.0 Stub implementation.
+	 * @since 6.7.0
 	 *
 	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
 	 *
@@ -1050,7 +1050,211 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 * @return bool Whether an element was found.
 	 */
 	private function step_in_head(): bool {
-		$this->bail( 'No support for parsing in the ' . WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD . ' state.' );
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		/*
+		 * > A character token that is one of U+0009 CHARACTER TABULATION,
+		 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+		 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+		 */
+		if ( '#text' === $op ) {
+			$text = $this->get_modifiable_text();
+			if ( '' === $text ) {
+				/*
+				 * If the text is empty after processing HTML entities and stripping
+				 * U+0000 NULL bytes then ignore the token.
+				 */
+				return $this->step();
+			}
+
+			if ( strlen( $text ) === strspn( $text, " \t\n\f\r" ) ) {
+				// Insert the character.
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+			}
+		}
+
+		switch ( $op ) {
+			/*
+			 * > 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 one of: "base", "basefont", "bgsound", "link"
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "meta"
+			 */
+			case '+META':
+				$this->insert_html_element( $this->state->current_token );
+
+				/*
+				 * > If the active speculative HTML parser is null, then:
+				 * >   - If the element has a charset attribute, and getting an encoding from
+				 * >     its value results in an encoding, and the confidence is currently
+				 * >     tentative, then change the encoding to the resulting encoding.
+				 */
+				$charset = $this->get_attribute( 'charset' );
+				if ( is_string( $charset ) ) {
+					$this->bail( 'Cannot yet process META tags with charset to determine encoding.' );
+				}
+
+				/*
+				 * >   - Otherwise, if the element has an http-equiv attribute whose value is
+				 * >     an ASCII case-insensitive match for the string "Content-Type", and
+				 * >     the element has a content attribute, and applying the algorithm for
+				 * >     extracting a character encoding from a meta element to that attribute's
+				 * >     value returns an encoding, and the confidence is currently tentative,
+				 * >     then change the encoding to the extracted encoding.
+				 */
+				$http_equiv = $this->get_attribute( 'http-equiv' );
+				$content    = $this->get_attribute( 'content' );
+				if (
+					is_string( $http_equiv ) &&
+					is_string( $content ) &&
+					0 === strcasecmp( $http_equiv, 'Content-Type' )
+				) {
+					$this->bail( 'Cannot yet process META tags with http-equiv Content-Type to determine encoding.' );
+				}
+
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "title"
+			 */
+			case '+TITLE':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noscript", if the scripting flag is enabled
+			 * > A start tag whose tag name is one of: "noframes", "style"
+			 *
+			 * The scripting flag is never enabled in this parser.
+			 */
+			case '+NOFRAMES':
+			case '+STYLE':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noscript", if the scripting flag is disabled
+			 */
+			case '+NOSCRIPT':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD_NOSCRIPT;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "script"
+			 *
+			 * @todo Could the adjusted insertion location be anything other than the current location?
+			 */
+			case '+SCRIPT':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "head"
+			 */
+			case '-HEAD':
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD;
+				return true;
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "html", "br"
+			 */
+			case '-BODY':
+			case '-HTML':
+			case '-BR':
+				/*
+				 * > Act as described in the "anything else" entry below.
+				 */
+				goto in_head_anything_else;
+				break;
+
+			/*
+			 * > A start tag whose tag name is "template"
+			 *
+			 * @todo Could the adjusted insertion location be anything other than the current location?
+			 */
+			case '+TEMPLATE':
+				$this->state->active_formatting_elements->insert_marker();
+				$this->state->frameset_ok = false;
+
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "template"
+			 */
+			case '-TEMPLATE':
+				if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+					// @todo Indicate a parse error once it's possible.
+					return $this->step();
+				}
+
+				$this->generate_implied_end_tags_thoroughly();
+				if ( ! $this->state->stack_of_open_elements->current_node_is( 'TEMPLATE' ) ) {
+					// @todo Indicate a parse error once it's possible.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( 'TEMPLATE' );
+				$this->state->active_formatting_elements->clear_up_to_last_marker();
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->reset_insertion_mode();
+				return true;
+		}
+
+		/*
+		 * > A start tag whose tag name is "head"
+		 * > Any other end tag
+		 */
+		if ( '+HEAD' === $op || $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 */
+		in_head_anything_else:
+		$this->state->stack_of_open_elements->pop();
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
 	}
 
 	/**
@@ -2991,7 +3195,117 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 	 * @return bool Whether an element was found.
 	 */
 	private function step_in_template(): bool {
-		$this->bail( 'No support for parsing in the ' . WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE . ' state.' );
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = $this->is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token
+			 * > A comment token
+			 * > A DOCTYPE token
+			 */
+			case '#text':
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+			case 'html':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is one of: "base", "basefont", "bgsound", "link",
+			 * > "meta", "noframes", "script", "style", "template", "title"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+			case '+META':
+			case '+NOFRAMES':
+			case '+SCRIPT':
+			case '+STYLE':
+			case '+TEMPLATE':
+			case '+TITLE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "colgroup", "tbody", "tfoot", "thead"
+			 */
+			case '+CAPTION':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is "col"
+			 */
+			case '+COL':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is "tr"
+			 */
+			case '+TR':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is one of: "td", "th"
+			 */
+			case '+TD':
+			case '+TH':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Any other start tag
+		 */
+		if ( ! $is_closer ) {
+			array_pop( $this->state->stack_of_template_insertion_modes );
+			$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+			$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+			return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Any other end tag
+		 */
+		if ( $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > An end-of-file token
+		 */
+		if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+			// Stop parsing.
+			return false;
+		}
+
+		// @todo Indicate a parse error once it's possible.
+		$this->state->stack_of_open_elements->pop_until( 'TEMPLATE' );
+		$this->state->active_formatting_elements->clear_up_to_last_marker();
+		array_pop( $this->state->stack_of_template_insertion_modes );
+		$this->reset_insertion_mode();
+		return $this->step( self::REPROCESS_CURRENT_NODE );
 	}
 
 	/**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 2830d87a81..83bb91ff91 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-alpha-58832';
+$wp_version = '6.7-alpha-58833';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.