The EXT_frag_depth extension is part of the WebGL API and enables to set a depth value of a fragment from within the fragment shader.

WebGL extensions are available using the WebGLRenderingContext.getExtension() method. For more information, see also Using Extensions in the WebGL tutorial.

Enable the extension:

gl.getExtension("EXT_frag_depth");

Now the output variable gl_FragDepthEXT is available to set a depth value of a fragment from within the fragment shader:

<script type="x-shader/x-fragment">
  void main() {
    gl_FragColor = vec4(1.0, 0.0, 1.0, 1.0);
    gl_FragDepthEXT = 0.5;
  }
</script>

• WebGLRenderingContext.getExtension()