Are CSS Line Grid  or CSS Step Sizing  addressing the problem of line-heights growing when sub/superscript is used ?
Wouldn’t the proposals create even more empty space around sub/superscripts? Could the proposals be fixed to address the sub/superscript problem?
Alternatively, would a fixed line height solve the problem? That was proposed earlier to look like this: line-height-style: variable | fixed . It would work great with sub/superscripts and could be used to create simple baseline grids, too. It would be simple for authors to understand and it would solve most problems authors have with line heights. Not all, but most.