A most excellent look at validation from the Daring Fireball. Personally I end up writing only XHTML nowadays if I can help it, which is XML, and should be valid XML, so I mainly just check for my documents being well formed XML, and use my own common sense to come as close to possible at making them valid, and then test them with the W3C validator. It works well enough for me. (Of course I don’t use BBEdit anymore…)
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