Prints a script to check whether or not the customizer is supported, and apply either the no-customize-support or customize-support class to the body.

This function MUST be called inside the body tag.

Ideally, call this function immediately after the body tag is opened. This prevents a flash of unstyled content.

It is also recommended that you add the “no-customize-support” class to the body tag by default.

Signature

wp_customize_support_script()

Return

(void)

Source

function wp_customize_support_script() {
	$admin_origin = parse_url( admin_url() );
	$home_origin  = parse_url( home_url() );
	$cross_domain = ( strtolower( $admin_origin[ 'host' ] ) != strtolower( $home_origin[ 'host' ] ) );

	?>
	<script type="text/javascript">
		(function() {
			var request, b = document.body, c = 'className', cs = 'customize-support', rcs = new RegExp('(^|\\s+)(no-)?'+cs+'(\\s+|$)');

<?php		if ( $cross_domain ): ?>
			request = (function(){ var xhr = new XMLHttpRequest(); return ('withCredentials' in xhr); })();
<?php		else: ?>
			request = true;
<?php		endif; ?>

			b[c] = b[c].replace( rcs, ' ' );
			b[c] += ( window.postMessage && request ? ' ' : ' no-' ) + cs;
		}());
	</script>
	<?php
}
WP Trac GitHub

Link here