Knowing that you are at the start of a context type seems generally useful for all context types, including TEXT/ATTRIBUTE/URL (we already have URL_START).
Instead of adding CSS_ATTRIBUTE_START, would it be better to add HtmlContext#isStart(), or event HtmlContext#getValueIndex()? http://gwt-code-reviews.appspot.com/1392801/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
