when using 'float: right' on the div element that, at that page, has class="tbar", the clas="tbar" div box overflows the boundaries of its parent div box. this also happens when the class="tbar" div does not have a padding specified on it (and when that div has only one element as its immediate child -- the 'span' element with the "Tioga" CDATA in it). In this situation, specifying a non-zero margin on the overflowing 'span' does not keep its box from overflowing that of its parent div element, though it seems that it should. some overflow can also happen as a result of border-widths the desired behavior: that the displayed box of the class="tbar" div would always contain the full displayed boxes of its children, including those boxes' borders (unless an offset was somehow explicitly specified [e.g.: with a negative margin value?] for those children, that would cause them to be positioned at least partially outside of the class="tbar" div)
that was with a local build of moz 0.9.5 on linux, by the way. and here's a screen-shot of what it looks on my box, using the mozilla components, in galeon: http://tioga.freeshell.org/proofs/mozilla-box-overflow.jpg [46 kb] the screen-shot is also linked from the example page at http://tioga.freeshell.org/proofs/mozilla-box-overflow.html
Invalid. The CSS2 spec is very clear on the fact that floating elements are removed from normal flow and do not affect the height of their ancestors. If you want the floating element to not overflow the ancestor, the ancestor must be positioned (just add position:relative to the stylesheet). As for border widths, that is also correct behavior if you are specifying "width:100%" or something along those lines and a border width.