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 8ccec5de029cc..734c3e38d094b 100644
--- a/src/wp-includes/html-api/class-wp-css-selectors.php
+++ b/src/wp-includes/html-api/class-wp-css-selectors.php
@@ -117,21 +117,31 @@ private static function parse( string $input ) {
 		$input = str_replace( array( "\r", "\f" ), "\n", $input );
 		$input = str_replace( "\0", "\u{FFFD}", $input );
 
-		$length    = strlen( $input );
-		$selectors = array();
-
 		$offset = 0;
 
-		while ( $offset < $length ) {
-			$selector = WP_CSS_ID_Selector::parse( $input, $offset );
-			if ( null !== $selector ) {
-				$selectors[] = $selector;
-			}
+		$selector = WP_CSS_Complex_Selector::parse( $input, $offset );
+		if ( null === $selector ) {
+			return null;
 		}
-		if ( count( $selectors ) ) {
-			return new WP_CSS_Selector_List( $selectors );
+		WP_CSS_Selector_Parser::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;
+			WP_CSS_Selector_Parser::parse_whitespace( $input, $offset );
+			$selector = WP_CSS_Complex_Selector::parse( $input, $offset );
+			if ( null === $selector ) {
+				return null;
+			}
+			$selectors[] = $selector;
+			WP_CSS_Selector_Parser::parse_whitespace( $input, $offset );
 		}
-		return null;
+
+		return new WP_CSS_Selector_List( $selectors );
 	}
 }
 
@@ -145,7 +155,7 @@ public static function parse( string $input, int &$offset );
 abstract class WP_CSS_Selector_Parser implements IWP_CSS_Selector_Parser {
 	const UTF8_MAX_CODEPOINT_VALUE = 0x10FFFF;
 
-	protected static function parse_whitespace( string $input, int &$offset ): bool {
+	public static function parse_whitespace( string $input, int &$offset ): bool {
 		$length   = strspn( $input, " \t\r\n\f", $offset );
 		$advanced = $length > 0;
 		$offset  += $length;
@@ -938,35 +948,38 @@ public static function parse( string $input, int &$offset ): ?self {
 
 		$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:
+			if (
+				self::COMBINATOR_CHILD === $input[ $updated_offset ] ||
+				self::COMBINATOR_NEXT_SIBLING === $input[ $updated_offset ] ||
+				self::COMBINATOR_SUBSEQUENT_SIBLING === $input[ $updated_offset ]
+			) {
 					$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;
+					// Failure to find a selector here is a parse error
+					$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;
+			} elseif ( ! $found_whitespace ) {
+				break;
+			} else {
+
+				/*
+				* Whitespace is ambiguous, it could be a descendant combinator or
+				* insignificant whitespace.
+				*/
+				$selector = WP_CSS_Selector::parse( $input, $updated_offset );
+				if ( null === $selector ) {
 					break;
+				}
+				$selectors[] = self::COMBINATOR_DESCENDANT;
+				$selectors[] = $selector;
 			}
-			// 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;
diff --git a/tests/phpunit/tests/html-api/wpCssSelectors.php b/tests/phpunit/tests/html-api/wpCssSelectors.php
index 4189ec586011a..33ada4ccbe3f9 100644
--- a/tests/phpunit/tests/html-api/wpCssSelectors.php
+++ b/tests/phpunit/tests/html-api/wpCssSelectors.php
@@ -357,15 +357,24 @@ public function test_parse_selector() {
 		$this->assertSame( ' > .child', substr( $input, $offset ) );
 	}
 
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_empty_selector() {
+		$input  = '';
+		$offset = 0;
+		$result = WP_CSS_Selector::parse( $input, $offset );
+		$this->assertNull( $result );
+	}
+
 	/**
 	 * @ticket TBD
 	 */
 	public function test_parse_complex_selector() {
-		$input  = 'el.foo#bar[baz=quux] > .child, rest';
+		$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 ) );
@@ -376,4 +385,58 @@ public function test_parse_complex_selector() {
 
 		$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 = WP_CSS_Complex_Selector::parse( $input, $offset );
+		$this->assertNull( $result );
+	}
+
+	public function test_parse_empty_complex_selector() {
+		$input  = '';
+		$offset = 0;
+		$result = WP_CSS_Complex_Selector::parse( $input, $offset );
+		$this->assertNull( $result );
+	}
+
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_selector_list() {
+		$input  = 'el.foo#bar[baz=quux] .descendent , rest';
+		$result = WP_CSS_Selector_List::from_selectors( $input );
+		$this->assertNotNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_selector_list() {
+		$input  = 'el,,';
+		$result = WP_CSS_Selector_List::from_selectors( $input );
+		$this->assertNull( $result );
+	}
+
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_selector_list2() {
+		$input  = 'el!';
+		$result = WP_CSS_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_Selector_List::from_selectors( $input );
+		$this->assertNull( $result );
+	}
 }