Looks like using browser sniffing, you can deliver an exaggerated Content-Length to everyone but Opera and browsers will deal with it gracefully. Pretty neat. (Obviously not desirable for violating the HTTP spec, but the UX gains might be worth it.)
For example major mobile operators pipe HTTP connections through a proxy that recompresses images. In that case you see e.g. Safari's or Opera's UA string, but you're actually dealing with proxy's HTTP behavior.
An older article (2008) that also talks about misreporting content-length for fun and profit: http://tech.hickorywind.org/articles/2008/05/23/content-leng...
Are suggesting they would rather guess the browser by inference through presence of DOM properties and methods?
In the end it narrows down to: would you rather a) break the functionality for some users in exchange to give the best possible solution to others, or b) give an OK experience to everyone
I usually go with "b". I think that frustration is much more powerful than awe.