How to reduce data loss in blending in WebGL - webgl

Question

I am working on a WebGL project that requires blending of 100 pictures. However, there is significant data loss in this process.
What I mean by that is, for example, I did a test to draw the same picture 100 times with each of them setting alpha value 1/100. In principle I should have got the same picture as itself but I have not.
I suspect that has something to do with framebuffer which only supports 8 bit unsign int array thus each time the picture gets drawn, its alpha value is truncated. I was wondering if I can use floating point texture with OES-texture-float extension and rendering to texture to solve the issue.
First I set up a framebuffer to render my 100 pictures to (after enabling the "OES-texture-float" extension). In particular, texImage2D takes the parameter gl.FlOAT instead of gl.UNSIGNED_BYTE. Finally I draw rttTexture as the final result to the display.
The issue is that when I used gl.UNSIGNED_BYTE it outputs a compromised result as I described before. However, when I used gl.FlOAT it outputs nothing. Is this the right process for me to solve this issue? Where might the problem be? (I am sure that the extension is enabled)
rttFramebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, rttFramebuffer);
rttFramebuffer.width = 512;
rttFramebuffer.height = 512;
rttTexture = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, rttTexture);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
gl.generateMipmap(gl.TEXTURE_2D);
//gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, rttFramebuffer.width rttFramebuffer.height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, rttFramebuffer.width, rttFramebuffer.height, 0, gl.RGBA, gl.FLOAT, null);
var renderbuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, renderbuffer);
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, rttFramebuffer.width, rttFramebuffer.height);
gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, rttTexture, 0);
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, renderbuffer);
gl.bindTexture(gl.TEXTURE_2D, null);
gl.bindRenderbuffer(gl.RENDERBUFFER, null);
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
gl.bindFramebuffer(gl.FRAMEBUFFER, rttFramebuffer);
drawScene();
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
drawFinalScene();