The answer lies in optimization. Eaglercraft is not streaming the game from a server (like GeForce Now); it is actually running the game logic locally. The developer(s) reverse-engineered the Minecraft protocol and rendering engine to compile it into a format that browsers understand.

Most Eaglercraft servers use the wss:// (WebSocket Secure) protocol. An example format is wss://://eaglercraft.com (note: specific 1.21 addresses vary by provider).

A middle school computer lab. The IT admin has blocked every gaming site, every proxy, and even renamed Minecraft.exe to System_Update_Required.exe . But Leo knows a secret.