diff --git a/src/wp-includes/html-api/class-wp-css-attribute-selector.php b/src/wp-includes/html-api/class-wp-css-attribute-selector.php
index be7332c85b72d..76ccdf3804b36 100644
--- a/src/wp-includes/html-api/class-wp-css-attribute-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-attribute-selector.php
@@ -1,7 +1,9 @@
 <?php
 
-final class WP_CSS_Attribute_Selector implements WP_CSS_HTML_Processor_Matcher {
-	public function matches( WP_HTML_Processor $processor ): bool {
+final class WP_CSS_Attribute_Selector implements WP_CSS_HTML_Tag_Processor_Matcher {
+	const WHITESPACE_CHARACTERS = " \t\r\n\f";
+
+	public function matches( WP_HTML_Tag_Processor $processor ): bool {
 		$att_value = $processor->get_attribute( $this->name );
 		if ( null === $att_value ) {
 			return false;
@@ -76,17 +78,17 @@ public function matches( WP_HTML_Processor $processor ): bool {
 	 * @return Generator<string>
 	 */
 	private function whitespace_delimited_list( string $input ): Generator {
-		$offset = strspn( $input, WP_CSS_Selector::WHITESPACE_CHARACTERS );
+		$offset = strspn( $input, self::WHITESPACE_CHARACTERS );
 
 		while ( $offset < strlen( $input ) ) {
 			// Find the byte length until the next boundary.
-			$length = strcspn( $input, WP_CSS_Selector::WHITESPACE_CHARACTERS, $offset );
+			$length = strcspn( $input, self::WHITESPACE_CHARACTERS, $offset );
 			if ( 0 === $length ) {
 				return;
 			}
 
 			$value   = substr( $input, $offset, $length );
-			$offset += $length + strspn( $input, WP_CSS_Selector::WHITESPACE_CHARACTERS, $offset + $length );
+			$offset += $length + strspn( $input, self::WHITESPACE_CHARACTERS, $offset + $length );
 
 			yield $value;
 		}
diff --git a/src/wp-includes/html-api/class-wp-css-class-selector.php b/src/wp-includes/html-api/class-wp-css-class-selector.php
index c4f858d4a05d9..c3e7ced008a6e 100644
--- a/src/wp-includes/html-api/class-wp-css-class-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-class-selector.php
@@ -1,7 +1,7 @@
 <?php
 
-final class WP_CSS_Class_Selector implements WP_CSS_HTML_Processor_Matcher {
-	public function matches( WP_HTML_Processor $processor ): bool {
+final class WP_CSS_Class_Selector implements WP_CSS_HTML_Tag_Processor_Matcher {
+	public function matches( WP_HTML_Tag_Processor $processor ): bool {
 		return (bool) $processor->has_class( $this->ident );
 	}
 
diff --git a/src/wp-includes/html-api/class-wp-css-complex-selector-list.php b/src/wp-includes/html-api/class-wp-css-complex-selector-list.php
new file mode 100644
index 0000000000000..f3769a035f6e5
--- /dev/null
+++ b/src/wp-includes/html-api/class-wp-css-complex-selector-list.php
@@ -0,0 +1,165 @@
+<?php
+/**
+ * HTML API: WP_CSS_Complex_Selector_List class
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @since TBD
+ */
+
+/**
+ * Core class used by the {@see WP_HTML_Processor} to parse and match CSS selectors.
+ *
+ * This class is designed for internal use by the HTML processor.
+ *
+ * For usage, see {@see WP_HTML_Processor::select()} or {@see WP_HTML_Processor::select_all()}.
+ *
+ * This class is instantiated via the {@see WP_CSS_Complex_Selector_List::from_selectors()} method.
+ * It takes a CSS selector string and returns an instance of itself or `null` if the selector
+ * is invalid or unsupported.
+ *
+ * A subset of the CSS selector grammar is supported. The grammar is defined in the CSS Syntax
+ * specification, which is available at {@link https://www.w3.org/TR/selectors/#grammar}.
+ *
+ * This class is rougly analogous to the <selector-list> in the grammar. See {@see WP_CSS_Compound_Selector_List} for more details on the grammar.
+ *
+ * This class supports the same selector syntax as {@see WP_CSS_Compound_Selector_List} as well as:
+ * - The following combinators:
+ *   - Next sibling (`el + el`)
+ *   - Subsequent sibling (`el ~ el`)
+ *
+ * @since TBD
+ *
+ * @access private
+ */
+class WP_CSS_Complex_Selector_List extends WP_CSS_Compound_Selector_List implements WP_CSS_HTML_Processor_Matcher {
+	/**
+	 * Takes a CSS selector string and returns an instance of itself or `null` if the selector
+	 * string is invalid or unsupported.
+	 *
+	 * @since TBD
+	 *
+	 * @param string $input CSS selectors.
+	 * @return static|null
+	 */
+	public static function from_selectors( string $input ) {
+		// > A selector string is a list of one or more complex selectors ([SELECTORS4], section 3.1) that may be surrounded by whitespace…
+		$input = trim( $input, " \t\r\n\r" );
+
+		if ( '' === $input ) {
+			return null;
+		}
+
+		/*
+		 * > The input stream consists of the filtered code points pushed into it as the input byte stream is decoded.
+		 * >
+		 * > To filter code points from a stream of (unfiltered) code points input:
+		 * > Replace any U+000D CARRIAGE RETURN (CR) code points, U+000C FORM FEED (FF) code points, or pairs of U+000D CARRIAGE RETURN (CR) followed by U+000A LINE FEED (LF) in input by a single U+000A LINE FEED (LF) code point.
+		 * > Replace any U+0000 NULL or surrogate code points in input with U+FFFD REPLACEMENT CHARACTER (�).
+		 *
+		 * https://www.w3.org/TR/css-syntax-3/#input-preprocessing
+		 */
+		$input = str_replace( array( "\r\n" ), "\n", $input );
+		$input = str_replace( array( "\r", "\f" ), "\n", $input );
+		$input = str_replace( "\0", "\u{FFFD}", $input );
+
+		$offset = 0;
+
+		$selector = self::parse_complex_selector( $input, $offset );
+		if ( null === $selector ) {
+			return null;
+		}
+		self::parse_whitespace( $input, $offset );
+
+		$selectors = array( $selector );
+		while ( $offset < strlen( $input ) ) {
+			// Each loop should stop on a `,` selector list delimiter.
+			if ( ',' !== $input[ $offset ] ) {
+				return null;
+			}
+			++$offset;
+			self::parse_whitespace( $input, $offset );
+			$selector = self::parse_complex_selector( $input, $offset );
+			if ( null === $selector ) {
+				return null;
+			}
+			$selectors[] = $selector;
+			self::parse_whitespace( $input, $offset );
+		}
+
+		return new self( $selectors );
+	}
+
+	/*
+	 * ------------------------------
+	 * Selector parsing functionality
+	 * ------------------------------
+	 */
+
+	/**
+	 * Parses a complex selector.
+	 *
+	 * > <complex-selector> = [ <type-selector> <combinator>? ]* <compound-selector>
+	 *
+	 * @return WP_CSS_Complex_Selector|null
+	 */
+	final protected static function parse_complex_selector( string $input, int &$offset ): ?WP_CSS_Complex_Selector {
+		if ( $offset >= strlen( $input ) ) {
+			return null;
+		}
+
+		$updated_offset = $offset;
+		$selector       = self::parse_compound_selector( $input, $updated_offset );
+		if ( null === $selector ) {
+			return null;
+		}
+
+		$selectors                       = array( $selector );
+		$has_preceding_subclass_selector = null !== $selector->subclass_selectors;
+
+		$found_whitespace = self::parse_whitespace( $input, $updated_offset );
+		while ( $updated_offset < strlen( $input ) ) {
+			if (
+				WP_CSS_Complex_Selector::COMBINATOR_CHILD === $input[ $updated_offset ] ||
+				WP_CSS_Complex_Selector::COMBINATOR_NEXT_SIBLING === $input[ $updated_offset ] ||
+				WP_CSS_Complex_Selector::COMBINATOR_SUBSEQUENT_SIBLING === $input[ $updated_offset ]
+			) {
+				$combinator = $input[ $updated_offset ];
+				++$updated_offset;
+				self::parse_whitespace( $input, $updated_offset );
+
+				// Failure to find a selector here is a parse error
+				$selector = self::parse_compound_selector( $input, $updated_offset );
+			} elseif ( $found_whitespace ) {
+				/*
+				* Whitespace is ambiguous, it could be a descendant combinator or
+				* insignificant whitespace.
+				*/
+				$selector = self::parse_compound_selector( $input, $updated_offset );
+				if ( null === $selector ) {
+					break;
+				}
+				$combinator = WP_CSS_Complex_Selector::COMBINATOR_DESCENDANT;
+			} else {
+				break;
+			}
+
+			if ( null === $selector ) {
+				return null;
+			}
+
+			// `div > .className` is valid, but `.className > div` is not.
+			if ( $has_preceding_subclass_selector ) {
+				throw new Exception( 'Unsupported non-final subclass selector.' );
+			}
+			$has_preceding_subclass_selector = null !== $selector->subclass_selectors;
+
+			$selectors[] = $combinator;
+			$selectors[] = $selector;
+
+			$found_whitespace = self::parse_whitespace( $input, $updated_offset );
+		}
+		$offset = $updated_offset;
+		return new WP_CSS_Complex_Selector( $selectors );
+	}
+}
diff --git a/src/wp-includes/html-api/class-wp-css-selector.php b/src/wp-includes/html-api/class-wp-css-compound-selector-list.php
similarity index 87%
rename from src/wp-includes/html-api/class-wp-css-selector.php
rename to src/wp-includes/html-api/class-wp-css-compound-selector-list.php
index 487c100ab47e4..2aae51d671f6b 100644
--- a/src/wp-includes/html-api/class-wp-css-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-compound-selector-list.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * HTML API: WP_CSS_Selector class
+ * HTML API: WP_CSS_Compound_Selector_List class
  *
  * @package WordPress
  * @subpackage HTML-API
@@ -8,18 +8,20 @@
  */
 
 /**
- * Core class used by the HTML processor to parse CSS selectors.
+ * Core class used by the {@see WP_HTML_Tag_Processor} to parse and match CSS selectors.
  *
- * This class is designed for internal use by the HTML processor.
+ * This class is designed for internal use by the HTML Tag Processor.
  *
- * This class is instantiated via the `WP_CSS_Selector::from_selectors( string $input )` method.
+ * For usage, see {@see WP_HTML_Tag_Processor::select()} or {@see WP_HTML_Tag_Processor::select_all()}.
+ *
+ * This class is instantiated via the {@see WP_CSS_Compound_Selector_List::from_selectors()} method.
  * It takes a CSS selector string and returns an instance of itself or `null` if the selector
  * is invalid or unsupported.
  *
  * A subset of the CSS selector grammar is supported. The grammar is defined in the CSS Syntax
  * specification, which is available at {@link https://www.w3.org/TR/selectors/#grammar}.
  *
- * This class is rougly analogous to the <selector-list> in the grammar. The supported grammar is:
+ * This class is analogous to <compound-selector-list> in the grammar. The supported grammar is:
  *
  *     <selector-list> = <complex-selector-list>
  *     <complex-selector-list> = <complex-selector>#
@@ -38,6 +40,10 @@
  *
  * @link https://www.w3.org/TR/selectors/#grammar Refer to the grammar for more details.
  *
+ * This class of selectors does not support "complex" selectors. That is any selector with a
+ * combinator such as descendent (`.ancestor .descendant`) or child (`.parent > .child`).
+ * See {@see WP_CSS_Complex_Selector_List} for support of some combinators.
+ *
  * Note that this grammar has been adapted and does not support the full CSS selector grammar.
  * Supported selector syntax:
  * - Type selectors (tag names, e.g. `div`)
@@ -50,12 +56,10 @@
  *   - child (`el > .child`)
  *
  * Unsupported selector syntax:
- * - Pseudo-element selectors (e.g. `::before`)
- * - Pseudo-class selectors (e.g. `:hover` or `:nth-child(2)`)
- * - Namespace prefixes (e.g. `svg|title` or `[xlink|href]`)
- * - The following combinators:
- *   - Next sibling (`el + el`)
- *   - Subsequent sibling (`el ~ el`)
+ * - Pseudo-element selectors (`::before`)
+ * - Pseudo-class selectors (`:hover` or `:nth-child(2)`)
+ * - Namespace prefixes (`svg|title` or `[xlink|href]`)
+ * - No combinators are supported (descendant, child, next sibling, subsequent sibling)
  *
  * Future ideas:
  * - Namespace type selectors could be implemented with select namespaces in order to
@@ -72,8 +76,12 @@
  * @link https://www.w3.org/TR/selectors-api2/
  * @link https://www.w3.org/TR/selectors-4/
  */
-class WP_CSS_Selector implements WP_CSS_HTML_Processor_Matcher {
-	public function matches( WP_HTML_Processor $processor ): bool {
+class WP_CSS_Compound_Selector_List implements WP_CSS_HTML_Tag_Processor_Matcher {
+	/**
+	 * @param WP_HTML_Tag_Processor $processor
+	 * @return bool
+	 */
+	public function matches( $processor ): bool {
 		if ( $processor->get_token_type() !== '#tag' ) {
 			return false;
 		}
@@ -87,14 +95,16 @@ public function matches( WP_HTML_Processor $processor ): bool {
 	}
 
 	/**
-	 * @var array<WP_CSS_Complex_Selector>
+	 * Array of selectors.
+	 *
+	 * @var array
 	 */
 	private $selectors;
 
 	/**
 	 * Constructor.
 	 *
-	 * @param array<WP_CSS_Complex_Selector> $selectors
+	 * @param array $selectors Array of selectors.
 	 */
 	protected function __construct( array $selectors ) {
 		$this->selectors = $selectors;
@@ -107,10 +117,9 @@ protected function __construct( array $selectors ) {
 	 * @since TBD
 	 *
 	 * @param string $input CSS selectors.
-	 * @return self|null
+	 * @return static|null
 	 */
-	public static function from_selectors( string $input ): ?self {
-		// > A selector string is a list of one or more complex selectors ([SELECTORS4], section 3.1) that may be surrounded by whitespace…
+	public static function from_selectors( string $input ) {
 		$input = trim( $input, " \t\r\n\r" );
 
 		if ( '' === $input ) {
@@ -132,7 +141,7 @@ public static function from_selectors( string $input ): ?self {
 
 		$offset = 0;
 
-		$selector = self::parse_complex_selector( $input, $offset );
+		$selector = self::parse_compound_selector( $input, $offset );
 		if ( null === $selector ) {
 			return null;
 		}
@@ -146,7 +155,7 @@ public static function from_selectors( string $input ): ?self {
 			}
 			++$offset;
 			self::parse_whitespace( $input, $offset );
-			$selector = self::parse_complex_selector( $input, $offset );
+			$selector = self::parse_compound_selector( $input, $offset );
 			if ( null === $selector ) {
 				return null;
 			}
@@ -391,73 +400,6 @@ final protected static function parse_compound_selector( string $input, int &$of
 		return null;
 	}
 
-	/**
-	 * Parses a complex selector.
-	 *
-	 * > <complex-selector> = [ <type-selector> <combinator>? ]* <compound-selector>
-	 *
-	 * @return WP_CSS_Complex_Selector|null
-	 */
-	final protected static function parse_complex_selector( string $input, int &$offset ): ?WP_CSS_Complex_Selector {
-		if ( $offset >= strlen( $input ) ) {
-			return null;
-		}
-
-		$updated_offset = $offset;
-		$selector       = self::parse_compound_selector( $input, $updated_offset );
-		if ( null === $selector ) {
-			return null;
-		}
-
-		$selectors                       = array( $selector );
-		$has_preceding_subclass_selector = null !== $selector->subclass_selectors;
-
-		$found_whitespace = self::parse_whitespace( $input, $updated_offset );
-		while ( $updated_offset < strlen( $input ) ) {
-			if (
-				WP_CSS_Complex_Selector::COMBINATOR_CHILD === $input[ $updated_offset ] ||
-				WP_CSS_Complex_Selector::COMBINATOR_NEXT_SIBLING === $input[ $updated_offset ] ||
-				WP_CSS_Complex_Selector::COMBINATOR_SUBSEQUENT_SIBLING === $input[ $updated_offset ]
-			) {
-				$combinator = $input[ $updated_offset ];
-				++$updated_offset;
-				self::parse_whitespace( $input, $updated_offset );
-
-				// Failure to find a selector here is a parse error
-				$selector = self::parse_compound_selector( $input, $updated_offset );
-			} elseif ( $found_whitespace ) {
-				/*
-				* Whitespace is ambiguous, it could be a descendant combinator or
-				* insignificant whitespace.
-				*/
-				$selector = self::parse_compound_selector( $input, $updated_offset );
-				if ( null === $selector ) {
-					break;
-				}
-				$combinator = WP_CSS_Complex_Selector::COMBINATOR_DESCENDANT;
-			} else {
-				break;
-			}
-
-			if ( null === $selector ) {
-				return null;
-			}
-
-			// `div > .className` is valid, but `.className > div` is not.
-			if ( $has_preceding_subclass_selector ) {
-				throw new Exception( 'Unsupported non-final subclass selector.' );
-			}
-			$has_preceding_subclass_selector = null !== $selector->subclass_selectors;
-
-			$selectors[] = $combinator;
-			$selectors[] = $selector;
-
-			$found_whitespace = self::parse_whitespace( $input, $updated_offset );
-		}
-		$offset = $updated_offset;
-		return new WP_CSS_Complex_Selector( $selectors );
-	}
-
 	/**
 	 * Parses a subclass selector.
 	 *
@@ -496,7 +438,7 @@ private static function parse_subclass_selector( string $input, int &$offset ) {
 	const UTF8_MAX_CODEPOINT_VALUE = 0x10FFFF;
 	const WHITESPACE_CHARACTERS    = " \t\r\n\f";
 
-	public static function parse_whitespace( string $input, int &$offset ): bool {
+	final public static function parse_whitespace( string $input, int &$offset ): bool {
 		$length   = strspn( $input, self::WHITESPACE_CHARACTERS, $offset );
 		$advanced = $length > 0;
 		$offset  += $length;
@@ -692,9 +634,9 @@ final protected static function parse_string( string $input, int &$offset ): ?st
 	 *
 	 * @param string $input
 	 * @param int $offset
-	 * @return string|null
+	 * @return string
 	 */
-	final protected static function consume_escaped_codepoint( $input, &$offset ): ?string {
+	final protected static function consume_escaped_codepoint( $input, &$offset ): string {
 		$hex_length = strspn( $input, '0123456789abcdefABCDEF', $offset, 6 );
 		if ( $hex_length > 0 ) {
 			/**
@@ -771,7 +713,7 @@ final protected static function consume_escaped_codepoint( $input, &$offset ): ?
 	 * @param int $offset The byte offset in the string.
 	 * @return bool True if the next two codepoints are a valid escape, otherwise false.
 	 */
-	private static function next_two_are_valid_escape( string $input, int $offset ): bool {
+	final protected static function next_two_are_valid_escape( string $input, int $offset ): bool {
 		if ( $offset + 1 >= strlen( $input ) ) {
 			return false;
 		}
@@ -858,7 +800,7 @@ final protected static function is_ident_codepoint( string $input, int $offset )
 	 * @param int $offset The byte offset in the string.
 	 * @return bool True if the next three codepoints would start an ident sequence, otherwise false.
 	 */
-	private static function check_if_three_code_points_would_start_an_ident_sequence( string $input, int $offset ): bool {
+	final protected static function check_if_three_code_points_would_start_an_ident_sequence( string $input, int $offset ): bool {
 		if ( $offset >= strlen( $input ) ) {
 			return false;
 		}
diff --git a/src/wp-includes/html-api/class-wp-css-compound-selector.php b/src/wp-includes/html-api/class-wp-css-compound-selector.php
index 1162aaef78c1e..e64695abe9ab3 100644
--- a/src/wp-includes/html-api/class-wp-css-compound-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-compound-selector.php
@@ -5,8 +5,8 @@
  *
  * > <compound-selector> = [ <type-selector>? <subclass-selector>* ]!
  */
-final class WP_CSS_Compound_Selector implements WP_CSS_HTML_Processor_Matcher {
-	public function matches( WP_HTML_Processor $processor ): bool {
+final class WP_CSS_Compound_Selector implements WP_CSS_HTML_Tag_Processor_Matcher {
+	public function matches( WP_HTML_Tag_Processor $processor ): bool {
 		if ( $this->type_selector ) {
 			if ( ! $this->type_selector->matches( $processor ) ) {
 				return false;
diff --git a/src/wp-includes/html-api/class-wp-css-id-selector.php b/src/wp-includes/html-api/class-wp-css-id-selector.php
index cc0589327c829..83339ff839317 100644
--- a/src/wp-includes/html-api/class-wp-css-id-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-id-selector.php
@@ -1,6 +1,6 @@
 <?php
 
-final class WP_CSS_ID_Selector implements WP_CSS_HTML_Processor_Matcher {
+final class WP_CSS_ID_Selector implements WP_CSS_HTML_Tag_Processor_Matcher {
 	/** @var string */
 	public $ident;
 
@@ -8,13 +8,14 @@ public function __construct( string $ident ) {
 		$this->ident = $ident;
 	}
 
-	public function matches( WP_HTML_Processor $processor ): bool {
+	public function matches( WP_HTML_Tag_Processor $processor ): bool {
 		$id = $processor->get_attribute( 'id' );
 		if ( ! is_string( $id ) ) {
 			return false;
 		}
 
-		$case_insensitive = method_exists( $processor, 'is_quirks_mode' ) && $processor->is_quirks_mode();
+		$case_insensitive = $processor->is_quirks_mode();
+
 		return $case_insensitive
 			? 0 === strcasecmp( $id, $this->ident )
 			: $processor->get_attribute( 'id' ) === $this->ident;
diff --git a/src/wp-includes/html-api/class-wp-css-type-selector.php b/src/wp-includes/html-api/class-wp-css-type-selector.php
index a2dcd16521cb5..c65adce14047d 100644
--- a/src/wp-includes/html-api/class-wp-css-type-selector.php
+++ b/src/wp-includes/html-api/class-wp-css-type-selector.php
@@ -1,7 +1,7 @@
 <?php
 
-final class WP_CSS_Type_Selector implements WP_CSS_HTML_Processor_Matcher {
-	public function matches( WP_HTML_Processor $processor ): bool {
+final class WP_CSS_Type_Selector implements WP_CSS_HTML_Tag_Processor_Matcher {
+	public function matches( WP_HTML_Tag_Processor $processor ): bool {
 		$tag_name = $processor->get_tag();
 		if ( null === $tag_name ) {
 			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 9f7a43acaebbd..bbca730279876 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -657,9 +657,14 @@ public function get_unsupported_exception() {
 	 * @param string $selector_string Selector string.
 	 * @return Generator<void> A generator pausing on each tag matching the selector.
 	 */
-	public function select_all( string $selector_string ): Generator {
-		$selector = WP_CSS_Selector::from_selectors( $selector_string );
+	public function select_all( $selector_string ): Generator {
+		$selector = WP_CSS_Complex_Selector_List::from_selectors( $selector_string );
 		if ( null === $selector ) {
+			_doing_it_wrong(
+				__METHOD__,
+				sprintf( 'Received unsupported or invalid selector "%s".', $selector_string ),
+				'6.8'
+			);
 			return;
 		}
 
@@ -692,7 +697,7 @@ public function select_all( string $selector_string ): Generator {
 	 * @param string $selector_string
 	 * @return bool  True if a matching tag was found, otherwise false.
 	 */
-	public function select( string $selector_string ) {
+	public function select( string $selector_string ): bool {
 		foreach ( $this->select_all( $selector_string ) as $_ ) {
 			return true;
 		}
diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index 7dadbc1bebdb2..a7633291b6bb2 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -860,6 +860,75 @@ public function change_parsing_namespace( string $new_namespace ): bool {
 		return true;
 	}
 
+	/**
+	 * Progress through a document pausing on tags matching the provided CSS selector string.
+	 *
+	 * @example
+	 *
+	 *     $processor = new WP_HTML_Tag_Processor(
+	 *         '<meta charset="utf-8"><title>Example</title><meta property="og:type" content="website"><meta property="og:description" content="An example.">'
+	 *     );
+	 *     foreach ( $processor->select_all( 'meta[property^="og:" i]' ) as $_ ) {
+	 *         // Loop is entered twice.
+	 *         var_dump(
+	 *             $processor->get_tag(),                   // string(4) "META"
+	 *             $processor->get_attribute( 'property' ), // string(7) "og:type" / string(14) "og:description"
+	 *             $processor->get_attribute( 'content' ),  // string(7) "website" / string(11) "An example."
+	 *         );
+	 *     }
+	 *
+	 * @since TBD
+	 *
+	 * @param string $selector_string Selector string.
+	 * @return Generator<void> A generator pausing on each tag matching the selector.
+	 */
+	public function select_all( $selector_string ): Generator {
+		$selector = WP_CSS_Compound_Selector_List::from_selectors( $selector_string );
+		if ( null === $selector ) {
+			_doing_it_wrong(
+				__METHOD__,
+				sprintf( 'Received unsupported or invalid selector "%s".', $selector_string ),
+				'6.8'
+			);
+			return;
+		}
+
+		while ( $this->next_tag() ) {
+			if ( $selector->matches( $this ) ) {
+				yield;
+			}
+		}
+	}
+
+	/**
+	 * Move to the next tag matching the provided CSS selector string.
+	 *
+	 * This method will stop at the next match. To progress through all matches, use
+	 * the {@see WP_HTML_Tag_Processor::select_all()} method.
+	 *
+	 * @example
+	 *
+	 *     $processor = new WP_HTML_Tag_Processor(
+	 *         '<meta charset="utf-8"><title>Example</title><meta property="og:title" content="Example">'
+	 *     );
+	 *     $processor->select( 'meta[charset]' );
+	 *     var_dump(
+	 *         $processor->get_tag(),                  // string(4) "META"
+	 *         $processor->get_attribute( 'charset' ), // string(5) "utf-8"
+	 *     );
+	 *
+	 * @since TBD
+	 *
+	 * @param string $selector_string
+	 * @return bool  True if a matching tag was found, otherwise false.
+	 */
+	public function select( string $selector_string ): bool {
+		foreach ( $this->select_all( $selector_string ) as $_ ) {
+			return true;
+		}
+		return false;
+	}
+
 	/**
 	 * Finds the next tag matching the $query.
 	 *
diff --git a/src/wp-includes/html-api/interface-wp-css-html-tag-processor-matcher.php b/src/wp-includes/html-api/interface-wp-css-html-tag-processor-matcher.php
new file mode 100644
index 0000000000000..73d108150bb95
--- /dev/null
+++ b/src/wp-includes/html-api/interface-wp-css-html-tag-processor-matcher.php
@@ -0,0 +1,8 @@
+<?php
+
+interface WP_CSS_HTML_Tag_Processor_Matcher {
+	/**
+	 * @return bool
+	 */
+	public function matches( WP_HTML_Tag_Processor $processor ): bool;
+}
diff --git a/src/wp-settings.php b/src/wp-settings.php
index 03c7a27fb9a80..76f47a9f6197d 100644
--- a/src/wp-settings.php
+++ b/src/wp-settings.php
@@ -265,6 +265,7 @@
 require ABSPATH . WPINC . '/html-api/class-wp-html-stack-event.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor-state.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor.php';
+require ABSPATH . WPINC . '/html-api/interface-wp-css-html-tag-processor-matcher.php';
 require ABSPATH . WPINC . '/html-api/interface-wp-css-html-processor-matcher.php';
 require ABSPATH . WPINC . '/html-api/class-wp-css-id-selector.php';
 require ABSPATH . WPINC . '/html-api/class-wp-css-type-selector.php';
@@ -272,7 +273,8 @@
 require ABSPATH . WPINC . '/html-api/class-wp-css-attribute-selector.php';
 require ABSPATH . WPINC . '/html-api/class-wp-css-compound-selector.php';
 require ABSPATH . WPINC . '/html-api/class-wp-css-complex-selector.php';
-require ABSPATH . WPINC . '/html-api/class-wp-css-selector.php';
+require ABSPATH . WPINC . '/html-api/class-wp-css-compound-selector-list.php';
+require ABSPATH . WPINC . '/html-api/class-wp-css-complex-selector-list.php';
 require ABSPATH . WPINC . '/class-wp-http.php';
 require ABSPATH . WPINC . '/class-wp-http-streams.php';
 require ABSPATH . WPINC . '/class-wp-http-curl.php';
diff --git a/tests/phpunit/tests/html-api/wpCssComplexSelectorList.php b/tests/phpunit/tests/html-api/wpCssComplexSelectorList.php
new file mode 100644
index 0000000000000..5b485a5029db5
--- /dev/null
+++ b/tests/phpunit/tests/html-api/wpCssComplexSelectorList.php
@@ -0,0 +1,107 @@
+<?php
+/**
+ * Unit tests covering WP_CSS_Complex_Selector_List functionality.
+ *
+ * @package WordPress
+ *
+ * @subpackage HTML-API
+ *
+ * @since TBD
+ *
+ * @group html-api
+ */
+class Tests_HtmlApi_WpCssComplexSelectorList extends WP_UnitTestCase {
+	private $test_class;
+
+	public function set_up(): void {
+		parent::set_up();
+		$this->test_class = new class() extends WP_CSS_Complex_Selector_List {
+			public function __construct() {
+				parent::__construct( array() );
+			}
+
+			public static function test_parse_complex_selector( string $input, int &$offset ) {
+				return self::parse_complex_selector( $input, $offset );
+			}
+		};
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_complex_selector() {
+		$input  = 'el1 > .child#bar[baz=quux] , rest';
+		$offset = 0;
+		$sel    = $this->test_class::test_parse_complex_selector( $input, $offset );
+
+		$this->assertSame( 3, count( $sel->selectors ) );
+
+		$this->assertSame( 'el1', $sel->selectors[2]->type_selector->ident );
+		$this->assertNull( $sel->selectors[2]->subclass_selectors );
+
+		$this->assertSame( WP_CSS_Complex_Selector::COMBINATOR_CHILD, $sel->selectors[1] );
+
+		$this->assertSame( 3, count( $sel->selectors[0]->subclass_selectors ) );
+		$this->assertNull( $sel->selectors[0]->type_selector );
+		$this->assertSame( 3, count( $sel->selectors[0]->subclass_selectors ) );
+		$this->assertSame( 'child', $sel->selectors[0]->subclass_selectors[0]->ident );
+
+		$this->assertSame( ', rest', substr( $input, $offset ) );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_complex_selector() {
+		$input  = 'el.foo#bar[baz=quux] > , rest';
+		$offset = 0;
+		$result = $this->test_class::test_parse_complex_selector( $input, $offset );
+		$this->assertNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_empty_complex_selector() {
+		$input  = '';
+		$offset = 0;
+		$result = $this->test_class::test_parse_complex_selector( $input, $offset );
+		$this->assertNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_complex_selector_list() {
+		$input  = 'el1 el2 el.foo#bar[baz=quux], second > selector';
+		$result = WP_CSS_Complex_Selector_List::from_selectors( $input );
+		$this->assertNotNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_selector_list() {
+		$input  = 'el,,';
+		$result = WP_CSS_Complex_Selector_List::from_selectors( $input );
+		$this->assertNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_selector_list2() {
+		$input  = 'el!';
+		$result = WP_CSS_Complex_Selector_List::from_selectors( $input );
+		$this->assertNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_empty_selector_list() {
+		$input  = " \t   \t\n\r\f";
+		$result = WP_CSS_Complex_Selector_List::from_selectors( $input );
+		$this->assertNull( $result );
+	}
+}
diff --git a/tests/phpunit/tests/html-api/wpCssSelector-parsing.php b/tests/phpunit/tests/html-api/wpCssCompoundSelectorList.php
similarity index 89%
rename from tests/phpunit/tests/html-api/wpCssSelector-parsing.php
rename to tests/phpunit/tests/html-api/wpCssCompoundSelectorList.php
index 4caa186158149..d94b61d49c14e 100644
--- a/tests/phpunit/tests/html-api/wpCssSelector-parsing.php
+++ b/tests/phpunit/tests/html-api/wpCssCompoundSelectorList.php
@@ -10,12 +10,12 @@
  *
  * @group html-api
  */
-class Tests_HtmlApi_WpCssSelector_Parsing extends WP_UnitTestCase {
+class Tests_HtmlApi_WpCssCompoundSelectorList extends WP_UnitTestCase {
 	private $test_class;
 
 	public function set_up(): void {
 		parent::set_up();
-		$this->test_class = new class() extends WP_CSS_Selector {
+		$this->test_class = new class() extends WP_CSS_Compound_Selector_List {
 			public function __construct() {
 				parent::__construct( array() );
 			}
@@ -51,10 +51,6 @@ public static function test_parse_compound_selector( string $input, int &$offset
 				return self::parse_compound_selector( $input, $offset );
 			}
 
-			public static function test_parse_complex_selector( string $input, int &$offset ) {
-				return self::parse_complex_selector( $input, $offset );
-			}
-
 			/*
 			 * Utilities
 			 */
@@ -402,53 +398,12 @@ public function test_parse_empty_selector() {
 		$this->assertSame( 0, $offset );
 	}
 
-	/**
-	 * @ticket TBD
-	 */
-	public function test_parse_complex_selector() {
-		$input  = 'el1 > .child#bar[baz=quux] , rest';
-		$offset = 0;
-		$sel    = $this->test_class::test_parse_complex_selector( $input, $offset );
-
-		$this->assertSame( 3, count( $sel->selectors ) );
-
-		$this->assertSame( 'el1', $sel->selectors[2]->type_selector->ident );
-		$this->assertNull( $sel->selectors[2]->subclass_selectors );
-
-		$this->assertSame( WP_CSS_Complex_Selector::COMBINATOR_CHILD, $sel->selectors[1] );
-
-		$this->assertSame( 3, count( $sel->selectors[0]->subclass_selectors ) );
-		$this->assertNull( $sel->selectors[0]->type_selector );
-		$this->assertSame( 3, count( $sel->selectors[0]->subclass_selectors ) );
-		$this->assertSame( 'child', $sel->selectors[0]->subclass_selectors[0]->ident );
-
-		$this->assertSame( ', rest', substr( $input, $offset ) );
-	}
-
-	/**
-	 * @ticket TBD
-	 */
-	public function test_parse_invalid_complex_selector() {
-		$input  = 'el.foo#bar[baz=quux] > , rest';
-		$offset = 0;
-		$result = $this->test_class::test_parse_complex_selector( $input, $offset );
-		$this->assertNull( $result );
-	}
-
-	public function test_parse_empty_complex_selector() {
-		$input  = '';
-		$offset = 0;
-		$result = $this->test_class::test_parse_complex_selector( $input, $offset );
-		$this->assertNull( $result );
-	}
-
-
 	/**
 	 * @ticket TBD
 	 */
 	public function test_parse_selector_list() {
-		$input  = 'el1 el2 el.foo#bar[baz=quux], rest';
-		$result = WP_CSS_Selector::from_selectors( $input );
+		$input  = 'el1, el2, el.foo#bar[baz=quux]';
+		$result = WP_CSS_Compound_Selector_List::from_selectors( $input );
 		$this->assertNotNull( $result );
 	}
 
@@ -457,7 +412,7 @@ public function test_parse_selector_list() {
 	 */
 	public function test_parse_invalid_selector_list() {
 		$input  = 'el,,';
-		$result = WP_CSS_Selector::from_selectors( $input );
+		$result = WP_CSS_Compound_Selector_List::from_selectors( $input );
 		$this->assertNull( $result );
 	}
 
@@ -466,7 +421,7 @@ public function test_parse_invalid_selector_list() {
 	 */
 	public function test_parse_invalid_selector_list2() {
 		$input  = 'el!';
-		$result = WP_CSS_Selector::from_selectors( $input );
+		$result = WP_CSS_Compound_Selector_List::from_selectors( $input );
 		$this->assertNull( $result );
 	}
 
@@ -475,7 +430,7 @@ public function test_parse_invalid_selector_list2() {
 	 */
 	public function test_parse_empty_selector_list() {
 		$input  = " \t   \t\n\r\f";
-		$result = WP_CSS_Selector::from_selectors( $input );
+		$result = WP_CSS_Compound_Selector_List::from_selectors( $input );
 		$this->assertNull( $result );
 	}
 }
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessor-select.php b/tests/phpunit/tests/html-api/wpHtmlProcessor-select.php
index c3a1e4121ecab..733a7135f1b17 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessor-select.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessor-select.php
@@ -66,4 +66,14 @@ public function test_select_all() {
 		}
 		$this->assertSame( 4, $count );
 	}
+
+	/**
+	 * @ticket TBD
+	 *
+	 * @expectedIncorrectUsage WP_HTML_Processor::select_all
+	 */
+	public function test_invalid_selector() {
+		$processor = WP_HTML_Processor::create_fragment( 'irrelevant' );
+		$this->assertFalse( $processor->select( '[invalid!selector]' ) );
+	}
 }
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor-select.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor-select.php
new file mode 100644
index 0000000000000..c42c69ff0a095
--- /dev/null
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor-select.php
@@ -0,0 +1,92 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Tag_Processor CSS selection functionality.
+ *
+ * Covers functionality related to CSS selectors and the {@see WP_HTML_Tag_Processor::select()}
+ * and {@see WP_HTML_Tag_Processor::select_all()} methods.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ *
+ * @since TBD
+ *
+ * @group html-api
+ */
+class Tests_HtmlApi_WpHtmlTagProcessor_Select extends WP_UnitTestCase {
+	/**
+	 * @ticket TBD
+	 */
+	public function test_select_miss() {
+		$processor = new WP_HTML_Tag_Processor( '<span>' );
+		$this->assertFalse( $processor->select( 'div' ) );
+	}
+
+	/**
+	 * @ticket TBD
+	 *
+	 * @dataProvider data_selectors
+	 */
+	public function test_select( string $html, string $selector ) {
+		$processor = new WP_HTML_Tag_Processor( $html );
+		$this->assertTrue( $processor->select( $selector ) );
+		$this->assertTrue( $processor->get_attribute( 'match' ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public static function data_selectors(): array {
+		return array(
+			'simple type'            => array( '<div match>', 'div' ),
+			'any type'               => array( '<html match>', '*' ),
+			'simple class'           => array( '<div class="x" match>', '.x' ),
+			'simple id'              => array( '<div id="x" match>', '#x' ),
+			'simple attribute'       => array( '<div att match>', '[att]' ),
+			'attribute value'        => array( '<div att="val" match>', '[att=val]' ),
+			'attribute quoted value' => array( '<div att="::" match>', '[att="::"]' ),
+
+			'list'                   => array( '<div><p match>', 'a, p' ),
+			'compound'               => array( '<div att><section att="foo bar" match>', 'section[att~="bar"]' ),
+		);
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_select_all() {
+		$processor = new WP_HTML_Tag_Processor( '<div match><p class="x" match><svg><rect match/></svg><i id="y" match></i>' );
+		$count     = 0;
+		foreach ( $processor->select_all( 'div, .x, rect, #y' ) as $_ ) {
+			++$count;
+			$this->assertTrue( $processor->get_attribute( 'match' ) );
+		}
+		$this->assertSame( 4, $count );
+	}
+
+	/**
+	 * @ticket TBD
+	 *
+	 * @expectedIncorrectUsage WP_HTML_Tag_Processor::select_all
+	 *
+	 * @dataProvider data_invalid_selectors
+	 */
+	public function test_invalid_selector( string $selector ) {
+		$processor = new WP_HTML_Tag_Processor( 'irrelevant' );
+		$this->assertFalse( $processor->select( $selector ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public static function data_invalid_selectors(): array {
+		return array(
+			'complex descendant' => array( 'div *' ),
+			'complex child'      => array( 'div > *' ),
+			'invalid selector'   => array( '[invalid!selector]' ),
+		);
+	}
+}