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Multiple Doctypes in a single HTML Document

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日久生厌
日久生厌 2021-01-18 05:29

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 s

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  • 深忆病人
    2021-01-18 05:37

    I believe the very first DOCTYPE is used by the browser and it's against the specification to have more than one in a document.

    I think (not sure) that the only situation when multiple DOCTYPE-s may be valid is when using IE conditional comments. Browsers other than IE won't see those, of course.

    I remember reading a blog entry (can't find it now, so I may be wrong in this) but some (most?) browsers even ignore the DOCTYPE if it's not the first thing they encounter. (This may have been a bug that got fixed since.)

    Here's W3School's reference page about DOCTYPE:

    http://www.w3schools.com/tags/tag_doctype.asp

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  • 日久生厌
    2021-01-18 05:47

    If you have multiple DOCTYPE-s in your HTML page then browser will consider first one, browser parse the DOM line by line. Once browser get DOCTYPE then it will stop looking for other doctypes and will jump to search for HTML tag.

    In the above question HTML-5 DOCTYPE is mentioned first and then HTML-4, according to this browser will render things as HTML-5 doctype

    It is better to try once in http://www.w3schools.com/ ... Try to use 'code' or 'kbd' or 'dfn' or 'samp' or 'strong' tag by mentioning both doctypes by priority.

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  • 清酒与你
    2021-01-18 06:00

    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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