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DOCTYPE declaration must be the very first thing in HTML page.
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<!doctyle html> is not a HTML tag but an instruction to browser about what version of HTML the page is written in.
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<!doctyle html> helps to parse and render the HTML in standard mode across the different browsers.
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If <!doctyle html> not provided, browser will render HTML page in quirks mode.
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<!doctype> is not case sensitive. That means this is also
<!DOCTYPE html>
also correct.