Here is the source code of the GUI for Delta Client. Basically this GUI is a Web-based mod and config manager for the Client. With support from Ultralight, an HTML rendering engine, the GUI runs perfectly inside the Java Client.
The DeltaClient.htm is the only file required for the GUI instance, which interacts with JavaScript API provided by Delta Client (Java). The test.js provides a virtual environment for testing the GUI in the Web browser. The lines in DeltaClient.htm that link test.js should be removed when running in the Web render powered by Ultralight inside the Java Client.
Delta Client was a Minecraft client for competitive PvP players. This project was created by @luanmenglei in early 2021 and has soon gone dead, leaving the code unfinished.
Please note that maintenance of the code is not considered.