I've implemented an embedded localhost web server demo for fpwebview. The demo runs on Linux macOS, and Windows 10. Here's a Linux screenshot:

The program consists of a multi-threaded web server listening on localhost:8000, and the web browser widget showing the content, styled by CSS, all loaded from said web server.

Because the embedded web server is listening on localhost:8000, you could visit it with a regular web browser running on the same machine. However, in the demo program, the buttons work by using webview APIs to perform what is effectively in-process RPC between Javascript and Pascal. From a regular web browser, clicking those buttons will have no effect, since the browser's Javascript runtime has nothing to do with the Pascal procedures implementing the buttons' functionalities.

For serious usage beyond a demo, additional security measures are possible to ensure that the embedded web server serves only the web browser widget and no other client.