WebGL does not support index drawing using index 255

Question

This is the simplest WebGL test program, which creates an index buffer with only one uint8 value of 255. It should draw a red square of size 64px, but it doesn't (the index value doesn't matter for drawing).

Somehow WebGL ignores the element at index 255 even though it is in the unsigned byte range. Using 254 or other values, the red square appears as expected.

Is this a bug in WebGL or expected behavior? I can't find any relevant information.

<canvas width="800" height="600"></canvas>
<script type="module">
let canvas = document.querySelector("canvas");
let gl = canvas.getContext("webgl2", {antialias: false});
let vert = gl.createShader(gl.VERTEX_SHADER);
let frag = gl.createShader(gl.FRAGMENT_SHADER);
let prog = gl.createProgram();
let index_buf = gl.createBuffer();

gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, index_buf);

gl.bufferData(
    gl.ELEMENT_ARRAY_BUFFER, new Uint8Array([255]), gl.STATIC_DRAW
);

gl.shaderSource(vert, `#version 300 es
    void main()
    {
        gl_Position = vec4(pos, 1);
        gl_PointSize = 64.0;
    }
`);

gl.shaderSource(frag, `#version 300 es
    precision highp float;
    out vec4 color;
    void main()
    {
        color = vec4(1,0,0,1);
    }
`);

gl.compileShader(vert);
gl.compileShader(frag);
gl.attachShader(prog, vert);
gl.attachShader(prog, frag);
gl.linkProgram(prog);
gl.clearColor(0, 0, 0, 1);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.useProgram(prog);
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, index_buf);
gl.drawElements(gl.POINTS, 1, gl.UNSIGNED_BYTE, 0);
</script>

Answer 1

View WebGL 2.0 Specification - PRIMITIVE_RESTART_FIXED_INDEX is always enabled :

Therefore, if the index type is UNSIGNED_BYTE, index 255 is the original restart index and cannot be used to index vertices.