diff --git a/src/wp-includes/html-api/class-wp-css-selectors.php b/src/wp-includes/html-api/class-wp-css-selectors.php
index 8d8ec35de98b6..8ccec5de029cc 100644
--- a/src/wp-includes/html-api/class-wp-css-selectors.php
+++ b/src/wp-includes/html-api/class-wp-css-selectors.php
@@ -123,9 +123,9 @@ private static function parse( string $input ) {
 		$offset = 0;
 
 		while ( $offset < $length ) {
-			$sel = WP_CSS_ID_Selector::parse( $input, $offset );
-			if ( $sel ) {
-				$selectors[] = $sel;
+			$selector = WP_CSS_ID_Selector::parse( $input, $offset );
+			if ( null !== $selector ) {
+				$selectors[] = $selector;
 			}
 		}
 		if ( count( $selectors ) ) {
@@ -841,6 +841,8 @@ public static function parse( string $input, int &$offset ): ?self {
 
 /**
  * This corresponds to <compound-selector> in the grammar.
+ *
+ * > <compound-selector> = [ <type-selector>? <subclass-selector>* ]!
  */
 final class WP_CSS_Selector extends WP_CSS_Selector_Parser {
 
@@ -856,12 +858,7 @@ private function __construct( ?WP_CSS_Type_Selector $type_selector, array $subcl
 	}
 
 	/**
-	 * Parses a selector string into a `WP_CSS_Selector` object.
-	 *
 	 * > <compound-selector> = [ <type-selector>? <subclass-selector>* ]!
-	 *
-	 * @param string $input The selector string to parse.
-	 * @return WP_CSS_Selector|null The parsed selector, or `null` if the selector is invalid or unsupported.
 	 */
 	public static function parse( string $input, int &$offset ): ?self {
 		if ( $offset >= strlen( $input ) ) {
@@ -882,6 +879,7 @@ public static function parse( string $input, int &$offset ): ?self {
 			$offset = $updated_offset;
 			return new self( $type_selector, $subclass_selectors );
 		}
+		return null;
 	}
 
 	/**
@@ -902,3 +900,76 @@ private static function parse_subclass_selector( string $input, int &$offset ) {
 			null ) );
 	}
 }
+
+
+/**
+ * This corresponds to <complex-selector> in the grammar.
+ *
+ * > <complex-selector> = <compound-selector> [ <combinator>? <compound-selector> ]*
+ */
+final class WP_CSS_Complex_Selector extends WP_CSS_Selector_Parser {
+	const COMBINATOR_CHILD              = '>';
+	const COMBINATOR_DESCENDANT         = ' ';
+	const COMBINATOR_NEXT_SIBLING       = '+';
+	const COMBINATOR_SUBSEQUENT_SIBLING = '~';
+
+	/**
+	 * even indexes are WP_CSS_Selector, odd indexes are string combinators.
+	 * @var array<WP_CSS_Selector>
+	 */
+	public $selectors = array();
+
+	private function __construct( array $selectors ) {
+		$this->selectors = $selectors;
+	}
+
+	public static function parse( string $input, int &$offset ): ?self {
+		if ( $offset >= strlen( $input ) ) {
+			return null;
+		}
+
+		$updated_offset = $offset;
+		$selector       = WP_CSS_Selector::parse( $input, $updated_offset );
+		if ( null === $selector ) {
+			return null;
+		}
+
+		$selectors = array( $selector );
+
+		$found_whitespace = self::parse_whitespace( $input, $updated_offset );
+		while ( $updated_offset < strlen( $input ) ) {
+			switch ( $input[ $updated_offset ] ) {
+				case self::COMBINATOR_CHILD:
+				case self::COMBINATOR_NEXT_SIBLING:
+				case self::COMBINATOR_SUBSEQUENT_SIBLING:
+					$combinator = $input[ $updated_offset ];
+					++$updated_offset;
+					self::parse_whitespace( $input, $updated_offset );
+					break;
+
+				default:
+					/*
+					 * Whitespace is a descendant combinator.
+					 * Either whitespace was found and we're on a selector,
+					 * or we've failed to find any combinator and parsing is complete.
+					 */
+					if ( ! $found_whitespace ) {
+						break 2;
+					}
+					$combinator = self::COMBINATOR_DESCENDANT;
+					break;
+			}
+			// Here we've found a combinator and need another selector.
+			$selector = WP_CSS_Selector::parse( $input, $updated_offset );
+			// Failure to find a selector is a parse error.
+			if ( null === $selector ) {
+				return null;
+			}
+			$selectors[]      = $combinator;
+			$selectors[]      = $selector;
+			$found_whitespace = self::parse_whitespace( $input, $updated_offset );
+		}
+		$offset = $updated_offset;
+		return new self( $selectors );
+	}
+}
diff --git a/tests/phpunit/tests/html-api/wpCssSelectors.php b/tests/phpunit/tests/html-api/wpCssSelectors.php
index 180bee4f53c05..4189ec586011a 100644
--- a/tests/phpunit/tests/html-api/wpCssSelectors.php
+++ b/tests/phpunit/tests/html-api/wpCssSelectors.php
@@ -347,13 +347,33 @@ public function test_parse_selector() {
 		$offset = 0;
 		$sel    = WP_CSS_Selector::parse( $input, $offset );
 
-		$this->assertSame( $sel->type_selector->ident, 'el' );
-		$this->assertSame( count( $sel->subclass_selectors ), 3 );
-		$this->assertSame( $sel->subclass_selectors[0]->ident, 'foo' );
-		$this->assertSame( $sel->subclass_selectors[1]->ident, 'bar' );
-		$this->assertSame( $sel->subclass_selectors[2]->name, 'baz' );
-		$this->assertSame( $sel->subclass_selectors[2]->matcher, WP_CSS_Attribute_Selector::MATCH_EXACT );
-		$this->assertSame( $sel->subclass_selectors[2]->value, 'quux' );
+		$this->assertSame( 'el', $sel->type_selector->ident );
+		$this->assertSame( 3, count( $sel->subclass_selectors ) );
+		$this->assertSame( 'foo', $sel->subclass_selectors[0]->ident, 'foo' );
+		$this->assertSame( 'bar', $sel->subclass_selectors[1]->ident, 'bar' );
+		$this->assertSame( 'baz', $sel->subclass_selectors[2]->name, 'baz' );
+		$this->assertSame( WP_CSS_Attribute_Selector::MATCH_EXACT, $sel->subclass_selectors[2]->matcher );
+		$this->assertSame( 'quux', $sel->subclass_selectors[2]->value );
 		$this->assertSame( ' > .child', substr( $input, $offset ) );
 	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_complex_selector() {
+		$input  = 'el.foo#bar[baz=quux] > .child, rest';
+		$offset = 0;
+		$sel    = WP_CSS_Complex_Selector::parse( $input, $offset );
+
+		var_dump( $sel );
+		$this->assertSame( 3, count( $sel->selectors ) );
+		$this->assertNotNull( $sel->selectors[0]->type_selector );
+		$this->assertSame( 3, count( $sel->selectors[0]->subclass_selectors ) );
+		$this->assertSame( WP_CSS_Complex_Selector::COMBINATOR_CHILD, $sel->selectors[1] );
+		$this->assertNull( $sel->selectors[2]->type_selector );
+		$this->assertSame( 1, count( $sel->selectors[2]->subclass_selectors ) );
+		$this->assertSame( 'child', $sel->selectors[2]->subclass_selectors[0]->ident );
+
+		$this->assertSame( ', rest', substr( $input, $offset ) );
+	}
 }