Traditional Forge or Fabric mods cannot be used because Eaglercraft is an AOT-compiled JavaScript version of Minecraft. Instead, users rely on specialized loaders and pre-modded clients: EaglerForge
These are distributed as a single Eaglercraft.html file. You don't install them – you just open the file.
Download or access a specific client (like Astro or Resent) through its official GitHub or web host.
Now go forth and mod responsibly—Eaglercraft awaits!