+++ This bug was initially created as a clone of Bug #28616 +++

We need a way to test support for various <link rel> features - e.g. prerender, prefetch, preload, preconnect.

Related threads:
- https://github.com/w3c/preload/issues/7
- https://lists.w3.org/Archives/Public/public-whatwg-archive/2015Apr/0000.html

Simon's proposal from the mailing list:

> For <link rel>, we could solve the feature-testing problem by normalizing the case for supported keywords but not unsupported keywords, so you can check with .rel or .relList:
>
> function preloadSupported() {
>    var link = document.createElement('link');
>    link.rel = 'PRELOAD';
>   return link.rel == 'preload';
> }