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Is it necessary to have valid CSS? I am using Twitter Bootstrap for rapid web development and after running the two main Bootstrap style sheets through the W3C CSS Validator, I noticed about 600 errors.
My question is, my site looks good so why is it so important for the CSS to be valid?
Yes, it is absolutely necessary to have valid CSS. CSS is a programming language and as such, you must follow its language rules. Just because browsers tend to accept invalid HTML and try to render it, it doesn’t make generating ill-formatted HTML a good practice. The same is true to CSS, although – fortunately – CSS rules are quite a bit stricter than HTML rules.
Another reason is that valid CSS has guaranteed behavior – if you write a rule, you can expect that rule to behave a certain way. (Buggy browsers, like all versions of IE aside.) If your CSS is invalid, any behavior you get is undefined and it may break when a patch release is issued for any of the browsers that you use for testing. Your CSS won’t necessarily break but when you write invalid CSS, you get no guarantees of any behavior – you simply get some behavior that may seem correct to you but that may change any time.
If you have correct CSS mixed in with incorrect CSS, browsers tend to ignore the invalid parts (just how the specification tells them to) but each browser does it slightly differently. Similarly, while many people advise to use CSS hacks, I’d say not to, for the above reasons.