Validate hash links when prerendering

[Rich-Harris]

Describe the problem

This is a relatively small thing, but it would be nice if the crawler could warn you if you prerender a page with a link like this...

<a href="/docs/routing#advanced">advanced routing</a>

...and the /docs/routing page was updated:

<h2 id="advanced-routing">Advanced routing</h2>

Describe the proposed solution

When prerendering, keep track of which hash links have been requested, and whether they match id attributes on the destination page.

Importance

nice to have

[mquandalle]

Would it be possible to generalize this hash link validation beyond pre-rendered pages?

[Rich-Harris]

In terms of runtime checking, no — the only way to see if a link is valid is to actually (attempt to) render the target. If we had a way of generating a typed route manifest, it might be possible to surface broken links, but that's a different problem and a very big can of worms.