Handle normalization when attributes starting with = follow boolean attributes

Author: dmsnell

src/wp-includes/html-api/class-wp-html-processor.php

@@ -1414,15 +1414,28 @@ public function serialize_token(): string {
 		}
 
 		$html .= "<{$qualified_name}";
+
+		$previous_attribute_was_true = false;
 		foreach ( $attribute_names as $attribute_name ) {
-			$html .= " {$this->get_qualified_attribute_name( $attribute_name )}";
+			$qualified_attribute_name = $this->get_qualified_attribute_name( $attribute_name );
+
+			if (
+				$previous_attribute_was_true &&
+				isset( $qualified_attribute_name[0] ) &&
+				'=' === $qualified_attribute_name[0]
+			) {
+				$html .= '=""';
+			}
+
+			$html .= " {$qualified_attribute_name}";
 			$value = $this->get_attribute( $attribute_name );
 
 			if ( is_string( $value ) ) {
 				$html .= '="' . htmlspecialchars( $value, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5 ) . '"';
 			}
 
-			$html = str_replace( "\x00", "\u{FFFD}", $html );
+			$previous_attribute_was_true = true === $value;
+			$html                        = str_replace( "\x00", "\u{FFFD}", $html );
 		}
 
 		if ( ! $in_html && $this->has_self_closing_flag() ) {