ShutterFly's a bad example (Score 1)

ShutterFly.com (linked in the News.com story) does user agent detection because it requires a browser with hooks into the OS (uploads, etc.). It's not warning-off certain browsers because page markup will break, but rather because the service won't work as advertised without browser access to the local file system -- and ShutterFly will have already taken your money before you find that out.

'Course ShutterFly.com's pages don't validate to their stated HTML 3.2 Final doctype either, so ...

Standards-compliant markup and liquid design are easy if you make them a priority. Their greatest enemy is pixel-perfect layouts, which are rarely necessary. Forget pixels, code relative, and be free.