fix (custom css): support nested css (#3199)

* fix: Ignore invalid CSS that affects the CSS of subsequent blocks

* fix: revert change

* fix: close any open curly braces

src/css-optimize.php

@@ -462,7 +462,17 @@ public static function generate_css( $styles ) {
 					// Optimize selectors by combining similar ones.
 					$selector_arr = self::combine_selectors( $selector_arr );
 
-					$css .= implode( ',', $selector_arr ) . $style_rules;
+					$combined_css = implode( ',', $selector_arr ) . $style_rules;
+
+					// Prevent invalid CSS in affecting the rest of the CSS
+					// by closing any open curly braces.
+					$open_curly_brace = substr_count( $combined_css , '{' );
+					$close_curly_brace = substr_count( $combined_css , '}' );
+					if ( $open_curly_brace > $close_curly_brace ) {
+						$combined_css .= str_repeat( '}', $open_curly_brace - $close_curly_brace );
+					}
+
+					$css .= $combined_css;
 				}
 
 				if ( ! empty( $media_query ) ) {