Define icon processing model for finding all icons of a document

[sideshowbarker]

…from #2200 (comment) with regard to potentially specifying standard UA requirements for apple-touch-icon (with sizes)

[domenic]

I think whether we put something in the standard depends crucially on whether more implementers than just Apple want to implement apple-touch-icon.

Also we should probably get Apple's opinion on this; I know they've been interested in unprefixing stuff, and might want to drop this in the future. @hober perhaps?

[annevk]

Previously: https://lists.w3.org/Archives/Public/public-whatwg-archive/2014Jul/thread.html#msg163

[domenic]

Some things to note, from the above thread:

• Chrome already supports this the apple-touch-icon link-rel. (Does Opera, I wonder?) So we have two engines.
• Chrome does not support the well-known URLs for apple-touch-icons.
• https://bugs.chromium.org/p/chromium/issues/detail?id=296962 is a Chrome bug to remove support, but there's some recent discussion in it pushing in the direction of speccing, instead of removing. (/cc @RByers)
• The above thread reminded me that not only do we have apple-touch-icon, but we have apple-touch-icon-precomposed.

Does anyone have a test page that only has apple-touch-icon (and maybe another test page with only apple-touch-icon-precomposed)? Then we could do some manual testing on e.g. Firefox and Opera and, if someone has a Windows Phone, Edge.