In dump.ly's use case, they would probably want client-side detection in JavaScript, not the UA string. You definitely have to be conservative implementing such an unexpected feature.
Client-side detection in JavaScript or UA string aren't mutually exclusive, since the UA string is exposed on the DOM.
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.
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...