Although, a common way to get them out of their "I know better" state is to point out the time factor and how much extra it will cost them. Suddenly they will worry over browser incompatibilities all day long.
The worse are the people that point out that the fifth paragraph on the third page ends with three words on the final line in the mockup but the website version has four words in Firefox and two words in Chrome. I just want to so tell them to just let the text flow its own way, dude. In the end I tell them in a nice way they're being silly.