This would be a breaking change because relative URLs are now relative to the client site URL. Maybe that's fine, though? Or perhaps we could ship a new resource type, like "blueprint-relative-url"?

This could be confusing, relative URLs are usually tied to the site context.
Using a new resource type would help, but it still feels like a workaround.

What if blueprints accepted bas64 encoded data? This would remove the need for putting the data on a server.
Playground could use a blueprint exporter and this could support storing data using base64.

The Playground could help some URLs by rewriting dynamically couldn't it? That is, recognize that it's a Github link, and if it's not the right rawusercontent domain then rewrite it?

Related: #18

I went for the CI job to rewrite branch URLs to trunk URLs:

fabe4d2

Independently from that, I will explore @dmsnell 's suggestion – it resolves another important problem.

To resolve this issue, we'll need to document the expected URL for all assets is the https://raw.githubusercontent.com/ URL pointing to the proposed branch.

Documented in d2f9f8d