Not all GitHub results are equal. To find a safe, working version of Eaglercraft 1.12:
Despite the difficulties, some developers have made progress on actual browser-based clients. eaglercraft 1.12 github
jadenacoder/Eaglercraft-1.12.2: This repository is the ... - GitHub Not all GitHub results are equal
Run the jar file once to generate the configuration files, and accept the eula.txt by changing its value to true . such as PlanetDogeCodes/Eagletcraft-1.12
On GitHub, you will find various "forks" or versions of the Eaglercraft 1.12 client, such as PlanetDogeCodes/Eagletcraft-1.12 , which is optimized to run on low-end hardware like Chromebooks.
Ensure your browser settings have hardware acceleration turned on so the game can utilize your dedicated or integrated graphics card.