Script Loader: Introduce HTML5 support for scripts and styles.

When a theme declares HTML5 support for script and styles via `add_theme_support( 'html5', array( 'script', 'style' ) )`, the `type="text/javascript"` and `type="text/css"` attributes are omitted.

These attributes are unnecessary in HTML5 and cause warnings in the W3C Markup Validation Service.

Props sasiddiqui, swissspidy, knutsp, SergeyBiryukov.
See #42804.

git-svn-id: https://develop.svn.wordpress.org/trunk@46164 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -1944,8 +1944,10 @@ function gallery_shortcode( $attr ) {
 	 *                    Otherwise, defaults to true.
 	 */
 	if ( apply_filters( 'use_default_gallery_style', ! $html5 ) ) {
+		$type_attr = current_theme_supports( 'html5', 'style' ) ? '' : ' type="text/css"';
+
 		$gallery_style = "
-		<style type='text/css'>
+		<style{$type_attr}>
 			#{$selector} {
 				margin: auto;
 			}