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Document: compatMode property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

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

Value

A string that is one of the following:

"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.

Examples

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

Specifications

Specification
DOM> # ref-for-dom-document-compatmode①>

Browser compatibility

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS
`compatMode`	8 1–8 Only supported for `HTMLDocument`, not all `Document` objects.	12	19 1–19 Only supported for `HTMLDocument`, not all `Document` objects.	≤12.1	5.1 3.1–5.1 Only supported for `HTMLDocument`, not all `Document` objects.	18	19 4–19 Only supported for `HTMLDocument`, not all `Document` objects.	≤12.1	5 2–5 Only supported for `HTMLDocument`, not all `Document` objects.	1.0	4.4	5 2–5 Only supported for `HTMLDocument`, not all `Document` objects.

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https://developer.mozilla.org/en-US/docs/Web/API/Document/compatMode