Comparing two ways to load non-critical CSS

noncritcss.md

I wanted to figure out the fastest way to load non-critical CSS so that the impact on initial page drawing is minimal.

TL;DR: Here's the solution I ended up with: https://github.com/filamentgroup/loadCSS/

---

For async JavaScript file requests, we have the async attribute to make this easy, but CSS file requests have no similar standard mechanism (at least, none that will still apply the CSS after loading - here are some async CSS loading conditions that do apply when CSS is inapplicable to media: https://gist.github.com/igrigorik/2935269#file-notes-md ).

Seems there are a couple ways to load and apply a CSS file in a non-blocking manner:

• A) Use an ordinary link element to reference the stylesheet, and place it at the end of the HTML document, after all the content. (This is what Google recommends here https://developers.google.com/speed/docs/insights/PrioritizeVisibleContent )
• B) Fetch it asynchronously with some inline JavaScript from the head of the page, by appending a link element to page dynamically. (To ensure it's async, I'm setting the link's media to a non-applicable media query, then toggling it back after the request goes out).

I suspected B would initiate the request sooner, since A would be dependent on the size of the content in the document - parsing it in entireity before kicking off the request.

In this case, sooner would be ideal to reduce impact on our page load process, since there could be minor reflows that are triggered by the styles in the non-critical CSS file (ideally, the critical CSS would have all reflow-triggering styles in it, but that so far, that's been hard to pull off across the breakpoints of a responsive design, and given that the initial inlined CSS should be kept very small to fit into the first round trip from the server).

I made some test pages, and they do seem to confirm that B will load the CSS sooner (B requests the css file after around 60-70ms whereas A usually requests it around 130-200ms). It's consistently about twice as fast.

• Demo A (static link at end of page): http://scottjehl.com/test/loadingcss/fromhtml.html
• Demo B (dynamically loaded link): http://scottjehl.com/test/loadingcss/fromscript.html

B comes with the additional benefits of qualifying the request based on whatever conditions we care to test too, which is nice.

• Update noted here: https://gist.github.com/scottjehl/87176715419617ae6994#comment-1243423
• Another note: @adactio helpfully pointed out that the approach would be a bit better if the non-critical CSS still loaded for non-JS environments. Wrapping a link to that file in a noscript tag and placing it at the end of the page would be a nice way to address that.
• Update: loadCSS has its own home now! https://github.com/filamentgroup/loadCSS/

I'm just curious, has anyone tried downloading the CSS as a script with type="text/css" and async?

<script id="styles" async type="text/css" src="style.css"></script>
<noscript><link rel="stylesheet" href="style.css"></noscript>

Surely that'd be non-blocking?
Then use JS to either rename the script tag to style, or copy its content across.
I tried something similar for overwriting critical-path-only CSS, which worked but was unused for other reasons.

For the record, Demo A loads the CSS faster than Demo B. For me, at least.

head

<script>
      (function() {
      'use strict';
      var head = document.getElementsByTagName('head')[0];
      var bootnap = document.createElement('link');
      bootnap.rel = 'stylesheet';
      bootnap.href = './css/bootstrap-theme1.min.css';
      head.appendChild(bootnap);
      }());
</script>

Before </body>

<link href='https://fonts.googleapis.com/css?family=Roboto:400,700' rel='stylesheet' type='text/css'>

Result:

---

head

<script>
// notice the difference between this one and the first one
      (function() {
          'use strict';
          var head = document.getElementsByTagName('script')[0];
          var bootnap = document.createElement('link');
          bootnap.rel = 'stylesheet';
          bootnap.href = './css/bootstrap-theme1.min.css';
          bootnap.media = 'only x';
          head.parentNode.insertBefore(bootnap, head);
          setTimeout(function() {
            bootnap.media = 'all';
          });
      }());
</script

Before </body>

<link href='https://fonts.googleapis.com/css?family=Roboto:400,700' rel='stylesheet' type='text/css'>

Result:

---

In the head

<script>
      (function() {
          'use strict';
          var head = document.getElementsByTagName('script')[0];
          var bootnap = document.createElement('link');
          bootnap.rel = 'stylesheet';
          bootnap.href = './css/bootstrap-theme1.min.css';
          bootnap.media = 'only x';
          head.parentNode.insertBefore(bootnap, head);
          setTimeout(function() {
            bootnap.media = 'all';
          });
      }());
</script>

Before </body>

<script>
      (function() {
          'use strict';
          var xhr = new XMLHttpRequest();
          xhr.timeout = 4000;
          xhr.overrideMimeType('text/css; charset=UTF-8');
          xhr.onreadystatechange = function() {
              if (xhr.readyState === 4 && xhr.status === 200) {
                  var style = document.createElement('style'),
                      lastJS = document.getElementsByTagName('script')[2];
                  style.appendChild(document.createTextNode(xhr.responseText));
                  lastJS.appendChild(style);
              }
          };
          xhr.open('GET', 'https://fonts.googleapis.com/css?family=Roboto:400,700', true);
          xhr.send(null);
      }());
</script>

Result:

---

The sweet spot (inlined critical above-the-fold css into the page itself):

Before </body>

<script>
    (function() {
        'use strict';
        var getAsyncFile = function(fileStr) {
            var xhr = new XMLHttpRequest();
            xhr.timeout = 4000;
            xhr.overrideMimeType('text/css; charset=UTF-8');
            xhr.onreadystatechange = function() {
                if (xhr.readyState === 4 && xhr.status === 200) {
                    var style = document.createElement('style'),
                        head = document.getElementsByTagName('head')[0];
                    style.appendChild(document.createTextNode(xhr.responseText));
                    head.appendChild(style);
                }
            };
            xhr.open('GET', fileStr, true);
            xhr.send(null);
        };
        getAsyncFile('./css/bootstrap-theme1.min.css');
        getAsyncFile('https://fonts.googleapis.com/css?family=Roboto:400,700');
</script>

Result:

The Desktop score went from 94 to 96 using the last example.

---

The browser is now fetching the bootstrap theme at 180ms, instead 220ms:

In the head

<script>
    (function(w) {
        'use strict';
        var xhrRunner = {
            firstRun: true
        };
        xhrRunner.getAsyncFile = function(fileStr) {
            var xhr = new XMLHttpRequest();
            xhr.timeout = 4000;
            xhr.overrideMimeType('text/css; charset=UTF-8');
            xhr.onreadystatechange = function() {
                if (xhr.readyState === 4 && xhr.status === 200) {
                    var style = document.createElement('style'),
                        head = document.getElementsByTagName('head')[0];
                    style.appendChild(document.createTextNode(xhr.responseText));
                    head.appendChild(style);
                }
            };
            xhr.open('GET', fileStr, true);
            xhr.send(null);
        };
        if (xhrRunner.firstRun) {
            xhrRunner.getAsyncFile('./css/bootstrap-theme1.min.css');
            xhrRunner.firstRun = false;
        }
        w.xhrRunner = xhrRunner;
    }(window));
</script>

Before </body>

<script>
    (function() {
        'use strict';
        xhrRunner.getAsyncFile(
        'https://fonts.googleapis.com/css?family=Roboto:400,700'
        );
    }());
</script>

Result:

---

Four days later:

Replaced the Roboto fonts with Helvetica.

Made seperate page that includes all the classes that my blog is using and used grunt-uncss. It looks like I've been using only 10% (13KB out of 126KB) of the bootstrap framework. The 13kb css is inlined into the page itself. My blog engine is separated on two parts, blog-engine 7.8kb and post-engine 75.5kb, instead serving one gigantic 90kb

I've developed and open sourced a new approach to handling asynchronous CSS and JS so I could use PhotoSwipe on my new blog. I hope you like it: https://www.npmjs.com/package/fetch-inject.