Document: compatMode property

The Document.compatMode read-only property indicates whether the document is rendered in Quirks mode or Standards mode.


An enumerated value that can be:

  • "BackCompat" if the document is in quirks mode.
  • "CSS1Compat" if the document is in no-quirks (also known as "standards") mode or limited-quirks (also known as "almost standards") mode.

Note: All these modes are now standardized, so the older "standards" and "almost standards" names are nonsensical and no longer used in standards.


if (document.compatMode === "BackCompat") {
  // in Quirks mode


DOM Standard
# ref-for-dom-document-compatmode①

Browser compatibility

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