Yeah, that's great, except that in the real world apps like Gmail have to support all kinds of wacky browsers, including old ones that get kicked to "legacy" UIs, mobile browsers, browsers that are technically standards compliant but are much faster or slower than other browsers and so on.
I used to work on a server that vended browser specific code based on the user-agent (for a variety of reasons it had to be browser specific choices on the server side). It was a server that vended some self contained code that got embedded into lots of different web sites and properties. Anyway, the most painful browser to support was by far Internet Explorer. It blew my mind how badly they managed to screw this up. It's not that modern IE's are bad browsers, you see, they aren't really - after letting the web rot for years they finally reacted to their retreating market share by staffing up the IE team again, and nowadays it can render things nice and fast. The problem is their totally broken compatibility architecture.
Combine this with trying to run code iframed into sites like Blogger where users are allowed to control their own toplevel HTML, and you can just forget about anything sane happening. But it gets even more confusing, because new versions of the rendering engine still have "quirks mode". You pretty quickly find yourself having to draw up giant matrices of how IE might behave in any given scenario.
What's worse, there are lots of different ways to ask IE for a specific mode. There are META tags, magic HTTP headers, DOCTYPE tags, and this Microsoft compatibility list which can override those in various situations, except that it works on a per domain basis and sites like google.com have tons of different apps hanging off different endpoints, some of which might no longer be really maintained, requiring a "flag day" where everyone co-ordinates to prepare for changes in the compatibility list. Oh yes, and users can and do modify their browser settings (as we see in this story), resulting in yet another column in the compatibility matrix.
Chrome, Firefox, Safari, Opera ... none of these browsers were such a nightmarish acid trip. Microsoft managed a seemingly impossible feat - dramatically improving the quality of their core rendering engine and yet STILL being the most horrible browser for web devs in existence! They snatched defeat from the claws of victory!