I would like to place a comment (
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Writing the DOCTYPE first is certainly best practice.
I remember strange problems a long, long time ago where some browser (probably IE6) ignored a DOCTYPE because there was something seemingly innocent before it - I think just whitespace, but maybe it was a comment. In any case, it was a horrible, horrible bug to have to track down, and there's certainly never any good reason to have comments or whitespace before the DOCTYPE.
Writing the DOCTYPE first is, I'd say, just something experienced web developers do to avoid horrible, elusive bugs.