Internet Explorer (of all things) already had the right idea with its "previous IE" mode: if the renderer emits standards errors while parsing the site,

Internet Explorer (of all things) already had the right idea with its "previous IE" mode: if the renderer emits standards errors while parsing the site, offer to retry with an older renderer (and send that renderer' original UA.) IE dropping into various modes to interpret your document can be wonderful when it works, but can also be a huge pain. I work on PageSpeed, an open source optimizing proxy, and this IE behavior keeps us from making a lot of optimizations on IE. Consider this code: <link rel=stylesheet href="main.css"> <!--[if IE 7]> <link rel=stylesheet href="ie.css"> <![endif]--> <link rel=stylesheet href="more.css"> Combining multiple CSS files into one is a great way to speed up page load times because most of the time in loading these resources is waiting for the round trip. We would like to issue: IE7: <link rel=stylesheet href="main+ie+more.css"> Other browsers: <link rel=stylesheet href="main+more.css"> But IE's impersonation keeps us from doing this. When we process a page requested with an IE UA, we don't know which version of IE it will decide to be when interpreting the page. Yes, it claims to be IE10, but if it sees something to drop it into IE7 mode then we shouldn't have removed ie.css. My favorite part of this announcement is the link [1] to where MS says this detection is going away: The next major version of IE will still honor document modes served by intranet sites, sites on the Compatibility View list, and when used with Enterprise Mode only. ... Public Internet sites will be rendered with the new Edge mode platform. [1] http://blogs.msdn.com/b/ie/archive/2014/11/11/living-on-the-edge-our-next-step-in-interoperability.aspx http://blogs.msdn.com/b/ie/archive/2014/11/11/living-on-the-...