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When CSS Blocks

February 27th, 2020

Tim Kadlec:

One particular pattern [for loading non-critical CSS] I’ve seen is the preload/polyfill pattern. With this approach, you load any stylesheets as preloads instead, and then use their onload events to change them back to a stylesheet once the browser has them ready.

So you’re trying to make your stylesheet more async, but it causes two big problems:

  1. You’ve kicked up the priority of the downloading higher than any other asset.
  2. You’ve blocked the HTML parser too (because of the polyfill as an inline script).

Firefox does something fancy to avoid problem #2 in this particular case, but it affects every other browser.

I’ve never had good luck with fancy techniques to trick the browser into theoretically better downloading/rendering patterns. I’m kind of a stylesheets in the head, scripts at the end of the body kinda guy, but I know the web is a complicated place. In fact, in a quick peek, I see that Jetpack is inserting an inline script into my <head>, so that would affect my loading too, except they load it with an obfuscated type until later scripts execute and change it, probably to avoid this exact problem.

Anyway, Tim’s advice:

• If you’re using loadCSS with the preload/polyfill pattern, switch to the print stylesheet pattern instead.

• If you have any external stylesheets that you’re loading normally (that is, as a regular stylesheet link), move any and all inline scripts that you can above it in the markup

• Inline your critical CSS for the fastest possible start render times.

The print pattern being:

<link rel="stylesheet" href="/path/to/my.css" media="print" onload="this.media='all'">