Subscripts and superscripts are very commonly used in chemistry, biology and physics. You can hardly write a sentence without needing one of those, e.g
- chemical formulas like H2O, H2SO4, H2 or O2
- isotopes like 13C or 238U
- variables or constants like τc or ΔrG°
Now, the major feature that makes it easier to type those things on the scientific SE sites is MathJax. This works very well for certain things in the question text, but it has significant drawbacks in other areas, like inside the question title.
Using MathJax in the title means that the LaTeX code is displayed e.g. in the Google result title, which is not exactly optimal. Using MathJax for chemical formulas is also not always appropriate as the different font and size used by it can make those formulas look a bit out of place.
So, in many cases using plain superscripts and subscripts would be preferable over MathJax. Though it could be vastly more convenient (e.g. with keyboard shortcuts), it is possible to already use them in the main body of a post. It is however not possible to use them in titles.
I propose to allow sub- and superscript tags in question titles, they should be rendered when the title is displayed on the site, and silently discarded for the HTML title and URLs.
The one case where this could get problematic is if somebody would actually want to talk about the
<sub> or the
<sup> tag. Either some way of escaping would be necessary, or it should be a per-site setting that is disabled on programming sites.