Async css

Allow css files to be marked as async meaning that they will not block rendering. The syntax could be a new attribute on the link element or a new value for "rel" attribute (that already have link types such as "dns-prefetch", "preconnect", "prefetch", and "preload" that also seems to serve a technical, how-to-load purpose), say "<link rel='stylesheet async' type='text/css' href='theme.css'>".

This functionality is similar to the async attribute on the script element, and the font-display descriptor for the @font-face CSS at-rule.

A web search will find quite a few articles about how to do this in a more or less hackish way, so there seems to be a demand for it. It seems that the browsers could rather easily add this in a much more reliable way as the browser e.g. knows if the resource is already in the cache and can control the priority of requests.

(I did read the contributing guidelines, but as I read them, they only talk about submitting pull requests and not about submitting issues)

Start by reading the issue discussion and the existing link types named in the proposal, then compare the async script attribute and the @font-face font-display descriptor. Done would require a settled specification for async stylesheet loading, including whether it uses a new link attribute or rel value and what browser behavior that entails.