Accessible (I Think) Split-Cell Table Headers

My colleague Chris Griffith, with whom I collaborated to put The Effects of Nuclear Weapons, Third Edition (1977) online, is also a spaceflight enthusiast (and an urban trails hiker: check out his new book!). He recently asked me how I would mark up a table with a split diagonal header cell; specifically, this one from the Apollo 16 documentation: The top left corner of the page in question. My immediate thought was to throw two spans in the header cell and position or grid them within that cell, but the accessibility of that seemed… questionable. It’s also what Wikipedia already does, and we here at meyerweb are nothing if not obsessed with finding new ways to do niche stuff. So I tried something different. But is its accessibility any better? If you want to see it as a live example, it’s over at Codepen. Most of the text in the table is what macOS Preview OCRed out of the original image, which I kept intact because I think it’s funny. Anyway, here was the original markup of the…