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Strictly speaking, a `STYLE` tag will be closed by a `STYLE` close tag. However, the CSS contents are concatenated together and the strict check for a STYLE close tag may later be invalidated by concatenation.

Author: sirreal

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

@@ -171,7 +171,7 @@ public function validate( $value ) {
 		 *
 		 * @see https://html.spec.whatwg.org/multipage/parsing.html#generic-raw-text-element-parsing-algorithm
 		 */
-		if ( preg_match( '#</style[ \\t\\f\\n\\r/>]#', $css ) ) {
+		if ( stripos( $css, '</style' ) ) {
 			$validity->add( 'illegal_markup', __( 'Markup is not allowed in CSS.' ) );
 		}
 

src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php

@@ -676,7 +676,7 @@ protected function validate_custom_css( $css ) {
 		 *
 		 * @see https://html.spec.whatwg.org/multipage/parsing.html#generic-raw-text-element-parsing-algorithm
 		 */
-		if ( preg_match( '#</style[ \\t\\f\\n\\r/>]#', $css ) ) {
+		if ( stripos( $css, '</style' ) ) {
 			return new WP_Error(
 				'rest_custom_css_illegal_markup',
 				__( 'Markup is not allowed in CSS.' ),