It would be great if one could run web server behind a proxy. Currently, it doesn't find assets correctly.
There are several options to solve this:

1. Using paths relative to the page where they are loaded from
2. Allow the user to set a prefix (this is the approach Coder follows with VSCODE_PROXY_URI environment variable and patch)

The current implementation of base path (#165391, #202491) is not sufficient as I explained in #202491 (comment) and following.