I don’t think shoving all of this complexity in the already incredibly complex OpenType text shaping engines is a good solution here. Your matrix example below

I don’t think shoving all of this complexity in the already incredibly complex OpenType text shaping engines is a good solution here. Your matrix example below is a good example of the can of worms that opens: there’s an explosion of internal state in the layout engine and a lot of things that Unicode has at least some success in defining (like “are these strings the same except for case” or “when is a set of codepoints a single character”) would become even harder and the set of badly handled corner cases would explode. It’s also unlikely you’d want to have it map that closely to Mathematica’s key bindings since those are designed to let you do things out of order and/or change things. In the end you’ll just be shifting who ends up refusing to implement your standard from the browser engine to a lower level.