MathML needs to work with vertical text.

What exactly does this mean? Is it a left-to-right block containing mathematics in a vertical line? I've asked some Japanese epub experts about this exact issue and was told they use left-to-right for mathematical texts. I'd love to see a sample (scan of a page) of vertical writing with modern mathematical text embedded.

Basically, you should rotate clockwise by 90 degrees for vertical-* values. Please take a look at spec: http://dev.w3.org/csswg/css3-writing-modes/#writing-mode saying: However replaced content involving text (such as MathML content or form elements) should match the replaced element's writing mode and line orientation if the UA supports such a vertical writing mode for the replaced content. I have some scan samples collected at: http://wiki.csswg.org/spec/utr50#math Hope this helps.