goog.userAgent.DOCUMENT_MODE

For IE version < 7, documentMode is undefined, so attempt to use the CSS1Compat property to see if we are in standards mode. If we are in standards mode, treat the browser version as the document mode. Otherwise, IE is emulating version 5.

NOTE(user): Support for IE < 7 is long gone, so this is now simplified. It returns document.documentMode for IE and undefined for everything else.