WebGL - reading pixel data from render buffer

Yes, you can read raw pixel data. Set preserveDrawingBuffer as true while getting webgl context and afterwards make use of readPixels by WebGL.

var context = canvasElement.getContext("webgl", {preserveDrawingBuffer: true}
var pixels = new Uint8Array(4 * width * height);
context.readPixels(x, y, width, height, context.RGBA, context.UNSIGNED_BYTE, pixels)

readPixels() should do what you want. Read more in the WebGL spec at http://www.khronos.org/registry/webgl/specs/latest/

This is very old question, but I have looked for the same in three.js recently. There is no direct way to render to frame buffer, but actually it is done by render to texture (RTT) process. I check the framework source code and figure out the following code:

renderer.render( rttScene, rttCamera, rttTexture, true );

// ...

var width = rttTexture.width;
var height = rttTexture.height;
var pixels = new Uint8Array(4 * width * height); // be careful - allocate memory only once

// ...

var gl = renderer.context;
var framebuffer = rttTexture.__webglFramebuffer;
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);        
gl.viewport(0, 0, width, height);
gl.readPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
gl.bindFramebuffer(gl.FRAMEBUFFER, null);