Complex sentence: In most browsers, the Document Object Model extension document.compatMode indicates the rendering mode for the current page--in standards mode and almost-standards mode, document.compatMode contains the value "CSS1Compat", while in quirks mode it equals "BackCompat".
Split into simpler sentences: In most browsers, the Document Object Model extension document.compatMode indicates the rendering mode for the current page. In standards mode and almost-standards mode, document.compatMode contains the value "CSS1Compat", while in quirks mode it equals "BackCompat".