404 page doesn’t render correctly on a subpath

[mbostock]

E.g., https://observablehq.com/framework/404

Since the 404 page can be served from anywhere, we need to know the root path.

[Fil]

maybe revive #194 (there is a whole discussion about why we probably didn't need it)

[mythmon]

I don't think there is anything I can do here. The link that Mike posted isn't hosted on Observable Cloud. I think the 404 page will need to be self contained somehow, either by using absolute links, or by being 100% self-contained. I don't know how to do the first while also not having dedicated builds for deploys.

[Fil]

OK I'll add a <base>