Since is a web-based version of Minecraft (specifically version 1.8.8), you cannot add mods using traditional methods like Forge or Fabric. Instead, "modding" Eaglercraft involves using Eaglercraft clients that come with pre-installed features or using EaglerXB (client-side plugins) .
Overview: You modify or replace the Eaglercraft client files (browser files or standalone) to include extra assets, client scripts, or altered code. This is the most common way to add visual changes, new UI, custom blocks or client-only features. how to add mods on eaglercraft
Once the proxy is connected to a standard Minecraft 1.8.8 server, you can add standard Spigot/Paper plugins to that server, and they will work for Eaglercraft players. Eaglercraft Since is a web-based version of Minecraft