When the writing-mode is set in the root element, currently there are warnings like: [7176] WARNING: have unconstrained inline-size; this should only result from very large sizes, not attempts at intrinsic inline-size calculation: 'AvailableISize() != NS_UNCONSTRAINEDSIZE', file c:\mozilla-source\mozilla-central\layout\generic\nsHTMLReflowState.cpp, line 360 [7176] WARNING: have unconstrained inline-size; this should only result from very large sizes, not attempts at intrinsic inline-size calculation: 'NS_UNCONSTRAINEDSIZE != computedISizeCBWM && NS_UNCONSTRAINEDSIZE != availISizeCBWM', file c:\mozilla-source\mozilla-central\layout\generic\nsHTMLReflowState.cpp, line 2384 [7176] WARNING: have unconstrained inline-size; this should only result from very large sizes, not attempts at intrinsic inline-size calculation: '(mFrameType == NS_CSS_FRAME_TYPE_INLINE && !frame->IsFrameOfType(nsIFrame::eReplaced)) || type == nsGkAtoms::textFrame || ComputedISize() != NS_UNCONSTRAINEDSIZE', file c:\mozilla-source\mozilla-central\layout\generic\nsHTMLReflowState.cpp, line 436

This is basically a trivial patch.