The Document.compatMode read-only property indicates whether the document is rendered in Quirks mode or Standards mode.
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.
if (document.compatMode === "BackCompat") {
// in Quirks mode
}