The EasyRPG Player for the Web wiki page is focused on how to deploy the webplayer in a website. If you want to modify the interpreter, you need to edit C++ code and build it with Emscripten. If you want to improve the “shell” (the index.html part displaying the canvas), there is a template in this source code path. Or just customize the existing generated index.html.
The EasyRPG buildscripts contain how to build the webplayer version with Emscripten. Build output may be checked in the Continuous Integration server.