Use position: absolute; or position: fixed; for both header and sidebar. Main area gets margin as big as header and sidebar. No framesets needed. That's at leas

Use position: absolute; or position: fixed; for both header and sidebar. Main area gets margin as big as header and sidebar. No framesets needed. That's at least what I did in 2001 and it worked like a charm. Even matching background elements of header and sidebar without any gaps.