Invalid. "CSS1Compat" merely means "Not violating CSS1". It does not mean "Not supporting CSS2". So we correctly apply all the _valid_ styles. Now your script is setting styles to invalid values. For example, it sets: tt.style.left = 100; when "counter" is "100". But "100" is not a valid value of "left"; "100px" would be. So we ignore the invalid value. See http://www.mozilla.org/docs/web-developer/quirks/ for an explanation of Mozilla's rendering modes and how these are determined based on doctype. "CSS1Compat" corresponds to either Standards or Almost Standards rendering mode (the string "CSS1Compat" was chosen for IE compatibility; see http://msdn.microsoft.com/workshop/author/dhtml/reference/properties/compatmode.asp for IE's documentation and http://www.mozilla.org/docs/dom/domref/dom_doc_ref11.html for the Mozilla documentation).