Fix: CSS minification & combination is not working for WordPress sites with mod_pagespeed or ngx_pagespeed

functions.php

<?php
/*Allowing mod_pagespeed or nginx_pagespeed to combine CSS files*/
/*By removing the ID field from the css link href*/
add_filter('style_loader_tag', function( $link ) {
  // Removing the Style ID from the WordPress CSS output as the ID in `link` causes the issue
  return preg_replace("/id='.*-css'/", "", $link);
} );