Describing WebGL as immediate mode is a little misleading. The rendering is immediate but the API is definitively not. There is a ton of state that needs to be allocated up front, mutated rather than re-created, and eventually torn down. Buffers, textures, shaders, and uniforms are all retained state. There's also the whole OpenGL state machine.

OpenGL used to have a true immediate mode where you called a function for each triangle vertex, and you didn't need shaders or buffers. That mode is not present in WebGL.