Document: compatMode property

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

Browser compatibility