You know: fonts, text sizes, and miscellaneous tweaks. I was thinking: we ship* some nice fonts (such as Source Sans), why not giving one of them some prominence, while we update those CSS files from 2004? Filing this bug so that I remember to get back to this. * In Windows, Mac, and official Linux builds. Of course I will preserve the fallback list for Linux distro builds that drop the fonts (e.g., Debian).
Adolfo Jayme Barrientos committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=106850a265b62c6a85d713c20afe131844013f0a tdf#82997 Update CSS for fonts used in systems of this decade
In the end I decided to just use the fonts of each platform’s UI (but I didn’t bother trying to specify macOS’ San Francisco in the CSS, because that requires hacks that may stop working anytime Apple desires to screw with people again).