PageSpeed: Remove unused CSS (deprecated)
Removing or deferring style rules that are not used by a document avoid downloads unnecessary bytes and allow the browser to start rendering sooner.
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Details from Google
Before a browser can begin to render a web page, it must download and parse any stylesheets that are required to lay out the page. Even if a stylesheet is in an external file that is cached, rendering is blocked until the browser loads the stylesheet from disk. In addition, once the stylesheet is loaded, the browser's CSS engine has to evaluate every rule contained in the file to see if the rule applies to the current page. Often, many web sites reuse the same external CSS file for all of their pages, even if many of the rules defined in it don't apply to the current page.
The best way to minimize the latency caused by stylesheet loading and rendering time is to cut down on the CSS footprint; an obvious way to do this is to remove or defer CSS rules that aren't actually used by the current page.