Moderators can edit the on-topic help page. (It's the only help page we can edit.)
As you can see, that page shows up as sans-serif:
However when editing the post preview uses our standard serif formatting we see on posts:
My expectation is the font would be sans-serif in the preview if it's sans-serif on the page.
This in particular gives some misleading impressions about what the heading formatting's going to look like. Left: nice and bold. Right: not.