Add subtle CSS styling for normative rule tags

[Timmmm]

Since #2195 we now have norm: tags. We probably want some nice styling for them so you can easily see them in the HTML output. An example from #1397

anchors.mp4

Actually there is the version that I did with subtle underlining in main already but the CSS is slightly wrong. I'll make a PR to fix that. Never mind, I misread git. Anyway here's how to get that underline version.