This is a great debate. I've thought about this quite a bit lately when the PageSpeed team was contacted by Microsoft [1] about the conflict between PageSpeed

This is a great debate. I've thought about this quite a bit lately when the PageSpeed team was contacted by Microsoft [1] about the conflict between PageSpeed assuming that Android 4.0 can handle webp, and mobile IE masquerading as Android thus breaking on PageSpeed enabled sites. The debate will never end but it's nice to understand the perspectives. PageSpeed, plus other WPO tools and complex sites, want the first view to be fast and small. We measure our effectiveness on first-view speedup. The most effective way to get this is by UA sniffing. We know about the other mechanisms and use them too at times, but they don't produce results that are as good for metrics we care about. However we are pretty serious about doing a good job about robust UA parsing; we put more energy into getting this right than a web developer should be expected to expend. The downside (as Microsoft points out) is that when we have to make an update we can't push it to all our users instantly. We should consider mechanisms to dynamically load an updated UA-mapping from a server we control when PageSpeed starts up, but haven't started such an effort. Browser vendors have their own legitimate motivations. Microsoft IE11 developers don't want to be punished for the shortcomings of earlier versions of IE, and want users of IE11 to get the best, most modern web sites possible, not the IE6 version. Microsoft justifiably wants servers to send mobile versions of sites to Windows Phones. I totally sympathize with their perspective. Chrome was in the same boat when it first launched so it has all kinds of masquerading in its UA and unfortunately still does. Same story, different decade. [1] https://code.google.com/p/modpagespeed/issues/detail?id=978 https://code.google.com/p/modpagespeed/issues/detail?id=978