A random fact from before the “before” in the article: the cache for all resource types used to be the same. We (ab)used this to do preloading before it was available in the web platform, e.g. download a JavaScript without executing it:

  var js = new Image();
  js.src = 'script.js';
  js.onerror = function(){/* js done */};

Then some browsers started having a separate image cache and this stopped working