The GitHub repo contains the EaglercraftX server software (Java-based) that allows you to host your own multiplayer server. This is how friends play together on school networks without touching Mojang's blocked servers.
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Double-click the downloaded HTML file. Your browser will open a new tab. You will see a loading screen (this loads the WebGL textures for the first time). Once finished, the Minecraft title screen will appear. The GitHub repo contains the EaglercraftX server software