Indicates whether the document is rendered in Quirks mode or Standards mode.


mode = document.compatMode 


  • "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.
Is an enumerated value that can be:

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


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


Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes ? Yes ? ? ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes ? Yes ? ? ?

Document Tags and Contributors

Last updated by: fscholz,