Cramped subscripts

MathML Core section 3.4.1.3 specifies cramped superscripts when the element has a computed CSS math-shift property equal to compact. The meaning of "cramped" being that the vertical alignment of a superscript is slightly lower than normal. TeX does the same thing, but it also cramps subscripts.

The lack of cramped subscripts causes MathML <sub> elements to be slightly taller than the TeX cramped equivalents. In most cases, I don't mind, but that size difference causes a visual problem with radicals because the algorithm sometimes picks a radical that is too tall. Example: $\sqrt{f_c'}$.

It seems that font designers have carefully chosen radical sizes to coordinate with commonly cramped (sub|super)script expressions. When the subscript is not cramped, the carefully designed radical does not fit and the algorithm picks the next larger radical size, which can be visually jarring. So I request a modification to section 3.4.1.2 that specifies cramped subscripts.

For clarification, be aware that the example I've chosen, $\sqrt{f_c'}$ contains two issues, not just this one. Naive MathML, such as currently implemented in GitHub threads, displays primes at a vertical alignment that is too high. That is issue #160. My Temml library takes steps to workaround that issue. Hopefully the prime problem will also be fixed someday but that is a separate problem from the lack of cramped subscripts.

Start with MathML Core section 3.4.1.2 and compare its treatment of subscripts with section 3.4.1.3's cramped superscript rules. Review the $\sqrt{f_c'}$ example while keeping issue #160 separate. Done means the specification clearly defines the requested cramped-subscript behavior without conflating the prime-alignment issue.