Express font size in px rather than em's

Different metafilter pages seem to encode base font-sizes differently,
e.g.
https://www.metafilter.com/101437/Large-earthquake-off-coast-of-Japan
https://metatalk.metafilter.com/20464/What-a-tangled-web-we-weave

Expressing font sizes for tooltips and links in em's has inconsistent
results. So express in px instead.