A straightforward post with some perf data from Tomas Pustelnik. It’s a good reminder that CSS is a crucial part of thinking web performance, and for a huge reason:
Any time [the browser] encounters any external resource (CSS, JS, images, etc.) it will assign it a download priority and initiate its download. Priorities are important because some resources are critical to render a page (eg. main stylesheet and JS files) while others may be less important (like images or stylesheets for other media types).
In the case of CSS, this priority is usually high because stylesheets are necessary to create CSSOM (CSS Object Model). To render a webpage browser has to construct both DOM and CSSOM.
That’s why CSS is often referred to as a “blocking” resource. That’s desirable to some degree: we wouldn’t want to see flash-of-unstyled-websites. But we get real perf gains when we make CSS smaller because it’s quicker to download, parse, and apply.