Browser developers care about being “WhatWG compliant”, because the WhatWG HTML group is composed by browser editors, they work with the W3C HTML5 group, and the HTML5 specification is based on the WhatWG’s HTML document.

Read this if you need more clarifications:

http://lists.w3.org/Archives/Public/public-whatwg-archive/20...

http://blog.whatwg.org/html-is-the-new-html5