I think the target of designing a website will be from any point of web designer to get it as close as possible can to comply with the W3C. Sometimes is hard to achieve this, working with css hacks, etc., but not impossible.
The true is if the main frame of the website comply with the W3C then no matter what the client does (50%), it should still look alright in all browsers.
From my own experience, some websites might look 100% perfect on IE, but checked and viewed on Firefox, was awfully all over the place.