> So if you occasionally forget and use http when you meant https and are worried about the consequences of that, you should just implement your own HSTS checking layer?

Or use a library to do it. The core fetch functionality shouldn't have to deal with HSTS. There may be legitimate reasons to fetch over HTTP even after you received an HSTS header - for testing purposes, for example.

> Why not just implement your own fetch wrapper that throws if it's not an https connection?

That's the developer dealing with HSTS.