The above mentioned validator lists the following as an 'advantage' over the W3C validator:
Quote:
The XML declaration must not be preceded by any characters. The W3C validator does not complain, this validator does
IE will not respect a doctype declaration unless it comes
before the xml declaration - in fact, before anything else on the page. Therefore, any page which is validated using this validator will send IE into quirks mode! This might be why the W3C validator is a little more lenient - it's all very well having a page which conforms perfectly, but if that causes the world's most popular browser to break, you just can't use it in the real world.
Of course, in a perfect world, all browsers would be fully compliant, and all render in exactly the same way (well, I can dream can't I?)

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