form input with HTML-CSS in 2.5+

I was updating https://github.com/mathjax/MathJax-docs/wiki/MathJax-and-form-input and it looks like the HTML-CSS output does not handle this trick well in 2.5+. Probably due to the dropped measurements. Not sure if there's an easy fix. SVG output and dpvc/chtml look good so not a high priority, I think.