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If a HTML document has two doctypes, how will the doctypes affect the rendering of the page and which doctype would the browser pick? Is having two (or more) doctypes in a single document valid or confusing?
Example:
<!DOCTYPE html>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<html>
</html>
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Only a single doctype declaration is permitted. This follows rather directly from the HTML specifications as well the HTML5 drafts, and it can also be checked using a validator.
Thus, there is no specification of what should happen. The natural expectation is that since browsers process the doctype declaration only in “doctype sniffing” when deciding on the browser mode (Quirks Mode vs. Standards Mode), only the first doctype declaration takes effect and the other is ignored.
This can be tested e.g. as follows (using an HTML 3.2 doctype, which triggers Quirks Mode on all doctype-sniffer browsers):
<!DOCTYPE HTML>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<title>Testing duplicate doctype effect</title>
<script>
document.write(document.compatMode);
</script>
</html>
This displays “CSS1Compat” (= Standards Mode), whereas swapping the doctype declarations causes “BackCompat” (= Quirks Mode).
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