How to specify the very basic project configuration?

[AnrDaemon]

$ npx parcel build app/index.html
🚨 Build failed.

@parcel/core: Failed to resolve '/jquery-ui.css' from './app/index.html'

  …\app\index.html:13:1
    12 |     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.
  > 13 |     <link rel="stylesheet" href="/jquery-ui.css">
  >    | ^
    14 |
    15 |     <title>…</title>

@parcel/resolver-default: Cannot load file './jquery-ui.css' in './'.
💡 Did you mean './app/jquery-ui.css'?

The file is located where it should. The ~/app/ of the repository is the webroot, and absolute references are relative to it.
It is not helping to start the parcel from within the app directory itself.

[AnrDaemon]

No idea? Should I step debug it myself?