[HTML-CSS, SVG] \underline misplaced in scriptstyle

In HTML-CSS and SVG output, the \underline macro does not place the underline properly. For example:

\scriptscriptstyle \underline{k}

shows no underline in HTML-CSS and misplaced underline in SVG output. CommonHTML shows the underline, but it is rather low.

This is originally from math.SE.