That's not the same as sending garbage code to the browser though. Yes they should do a query for supported html5 fucntions and deliver the page the browser can support instead of relying on antiquated user agents.
However they have not been as bastardly about it as google have. like when they took over google docs many an eon ago. the version they took over worked in all three major browsers at the time, IE, FF and opera, just fine in fact. Shortly after google took over(this was when IE had 90% marketshare and had to be supported and Google was sponsoring FF mind you) They did exactly two changes to the service in their first patch. they changed the branding to google, and they changed it so Opera users got a message they could use the service as it wasn't supported. if you tried using it, several things was suddenly broken, change user ID and it worked. shortly thereafter there was a new "update" that again didn't actually change anything on the surface, but they actually added a fucntion that went beyond the user agent to check the browser and would send the same garbage code to opera again, since this wasn't a simple user agent fix, it was a lot harder to spoof.
upholding their do no evil mantra, I'm sure they had honest intentions