Standard Minecraft uses the LWJGL (Lightweight Java Game Library) library, which utilizes OpenGL for graphics rendering. A web port must replace these backend rendering calls. Developers use WebGL (Web Graphics Library), a JavaScript API for rendering interactive 2D and 3D graphics within any compatible web browser without the use of plug-ins. The porting process involves creating a compatibility layer that translates OpenGL function calls into WebGL calls.
The single-player mode is nearly identical to vanilla Minecraft 1.5.2. You have: eaglercraft 115 portable