wp/app/themes/twentyfifteen/js/skip-link-focus-fix.js

/**
 * Makes "skip to content" link work correctly in IE9, Chrome, and Opera
 * for better accessibility.
 *
 * @link http://www.nczonline.net/blog/2013/01/15/fixing-skip-to-content-links/
 */

( function() {
	var is_webkit = navigator.userAgent.toLowerCase().indexOf( 'webkit' ) > -1,
	    is_opera  = navigator.userAgent.toLowerCase().indexOf( 'opera' )  > -1,
	    is_ie     = navigator.userAgent.toLowerCase().indexOf( 'msie' )   > -1;

	if ( ( is_webkit || is_opera || is_ie ) && document.getElementById && window.addEventListener ) {
		window.addEventListener( 'hashchange', function() {
			var element = document.getElementById( location.hash.substring( 1 ) );

			if ( element ) {
				if ( ! /^(?:a|select|input|button|textarea)$/i.test( element.tagName ) ) {
					element.tabIndex = -1;
				}

				element.focus();
			}
		}, false );
	}
} )();