In your stylesheet:
div#content.myClass
Edit: These might help, too:
div#content.myClass.aSecondClass.aThirdClass /* Won't work in IE6, but valid */
div.firstClass.secondClass /* ditto */
and, per your example:
div#content.sectionA
Edit, 4 years later: Since this is super old and people keep finding it: don’t use the tagNames in your selectors. #content.myClass
is faster than div#content.myClass
because the tagName adds a filtering step that you don’t need. Use tagNames in selectors only where you must!