Conditional comments are totally valid markup (for HTML as well as XHTML doctypes) that cause IE browsers running in Windows to parse the contents of the commented markup.
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How to target specific IE browsers using just HTML?
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How do I choose a Doctype?
When should you choose a transitional doctype over a strict one? What is the correct usage of the frameset doctype? This post gives you all the details you need to choose the right doctype for your web site.