cssrelpreload.js

/*! loadCSS rel=preload polyfill. [c]2017 Filament Group, Inc. MIT License */
(function( w ){
  // rel=preload support test
  if( !w.loadCSS ){
    return;
  }
  var rp = loadCSS.relpreload = {};
  rp.support = function(){
    try {
      return w.document.createElement( "link" ).relList.supports( "preload" );
    } catch (e) {
      return false;
    }
  };

  // loop preload links and fetch using loadCSS
  rp.poly = function(){
    var links = w.document.getElementsByTagName( "link" );
    for( var i = 0; i < links.length; i++ ){
      var link = links[ i ];
      if( link.rel === "preload" && link.getAttribute( "as" ) === "style" ){
        w.loadCSS( link.href, link, link.getAttribute( "media" ) );
        link.rel = null;
      }
    }
  };

  // if link[rel=preload] is not supported, we must fetch the CSS manually using loadCSS
  if( !rp.support() ){
    rp.poly();
    var run = w.setInterval( rp.poly, 300 );
    if( w.addEventListener ){
      w.addEventListener( "load", function(){
        rp.poly();
        w.clearInterval( run );
      } );
    }
    if( w.attachEvent ){
      w.attachEvent( "onload", function(){
        w.clearInterval( run );
      } )
    }
  }
}( this ));