When adding superscripts and/or subscripts to a text, the line height of the line increases.
This leads to uneven line spacing between lines which
- is very bad typographic style
- makes it hard (if not impossible) to distinguish between spacing produced by new paragraphs and spacing produced because of the reported issue.
- simply looks ugly
Especially point 2) is detrimental for all Wikipedia projects since it conceals the intended structure of the text and therefore makes texts and content harder to understand and to follow.
I know, that this is basically a browser issue (I tested Firefox 21 as well as Internet Explorer 10 on Windows 7 with Vector skin) and not MediaWiki's fault. However I think we should consider adding some workaround to fix the bad rendering of sub- and supscripts in modern browsers.
A quick fix I tested would be changing the <sup> and <sub> tags style to include "line-height:0;". This makes texts look reasonable, with the downside that (with the current default line height of the surrounding text) subscripts would touch superscripts of the following line if they happened to be at the same position (very unlikely though). This could be easily fixed by increasing the default line-height, though.
However, I'm no specialist with CSS, so it's likely there could be a better solution around.
I hope we can find a solution for this issue. It's especially annoying in scientific articles on chemical, physical or mathematical topics, where many super- and subscript are used. In some articles there are fewer lines with the intended line height than there are with increased line height.