You cannot run Forge mods, but you can customize the HTML file itself if you have technical skills.
Many Eaglercraft 1.15 portable builds allow you to save your world locally to the browser's cache or export the file, meaning you can play without an active internet connection once the page is loaded. How to Get Started eaglercraft 115 portable
Because Eaglercraft 1.15 Portable is a reimplementation, not an emulation, some features behave differently or are missing. You cannot run Forge mods, but you can
Once you have the file, follow these steps to make it truly portable: Create a Folder You cannot run Forge mods