HTML - doctype


The doctype is the first line of a xml document that defines its type.

Html doctype

For a html document, it's:

<!DOCTYPE html>

Parser may require an EOL character


It's mandatory otherwise the browser will turn into quirks mode 1) and some library will not work as expected 2).

How to check?


If you do a lighthouse check, it's also reported.

Lighthouse Doctype Mandatory

Javascript console

With javascript, you can check the mode with the compat mode


If its value is:

  • BackCompat, the document is in quirks mode.
  • CSS1Compat, it isn't

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XML - doctype

doctype is a DTD schema definition. It MUST appear before the first element in the document. The HTML doctype: Example of attribute declarations ...

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