Implement columnspan for MathML’s mtd element

Created by: gimsieke

I see there has been some discussion, but no progress on this matter. A customer of ours intends to use MathJax for presentational MathML rendering. There are some aligned expressions in their content, currently expressed using mtd/@columnspan, since this works in Firefox’s native MathML implementation at least. <maligngroup/> does not seem to be supported by both implementations. I’m aware of #1104 (closed). Is there any chance that either alignment markers or column/row spanning attributes will be supported any time soon? Will it accelerate development if the customer provides some funding?