Maybe I'm being pedantic, but the HTML reference to the favicon is the convention. I personally roll my eyes when I deploy something and see tons of 404s in my log for favicons that I've not made available or suggested that browsers attempt to load.
>HTML reference to the favicon is the convention //
No, the convention is that a favicon.ico file is found in the root of the site and that you can instead locate an icon by supplying a URL. Sucks as a convention.
Interestingly Google on their search homepage (for me at least) are using a microdata markup and not serving an explicit icon:
