CSS was designed for vendor extension. This is why browsers are supposed to ignore selectors they do not understand. Mozilla and Opera use this too. What you

CSS was designed for vendor extension. This is why browsers are supposed to ignore selectors they do not understand. Mozilla and Opera use this too. What you are comparing this to is different: a browser that does the wrong thing with a standardized selector. And, of course, HTML violations are in a different class, as it does not have extensibility built into the spec like CSS has.