I think you mean "about 0%" see a broken site in response to my comment. Anyway, I get your point. However, I am a big fan of the robustness principle: Be cons

I think you mean "about 0%" see a broken site in response to my comment. Anyway, I get your point. However, I am a big fan of the robustness principle: Be conservative in what you do, be liberal in what you accept from others. So for me personally, the issue is clear-cut. I will keep my codebase as clean as possible/affordable, be it HTML or C++, not relying on quirks that have been introduced to tolerate faults of careless designers and programmers.