Implement HTMLLinkElement#relList and HTMLLinkElement#relList.supports
In order to feature detect resource hinting rels like prefetch, preload, and dns-prefetch, please implement relList and relList.supports as added to the dom spec in https://github.com/whatwg/dom/pull/123 . relList.supports is implemented in current Firefox and Chrome browsers. It is not yet implemented in Safari, but the resource hinting is also not implemented, so feature testing for that is a true negative. However, Edge supports resource hinting, but feature testing is a false negative, which means developers have to resort to User Agent inference or forgo resource hinting, resulting in a slower experience for users.
This has been implemented and is working its way through our development branches.