readPixels() with format RGB outputs zero

[UlyssesZh]

const width = 4;
const height = 4;
const gl = require('gl')(width, height, {preserveDrawingBuffer: true});
gl.clearColor(1, 0, 0, 1);
gl.clear(gl.COLOR_BUFFER_BIT);

const pixelsRgba = new Uint8Array(width * height * 4);
gl.readPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixelsRgba);
console.log(pixelsRgba);

const pixelsRgb = new Uint8Array(width * height * 3);
gl.readPixels(0, 0, width, height, gl.RGB, gl.UNSIGNED_BYTE, pixelsRgb);
console.log(pixelsRgb);

Output:

Uint8Array(64) [
  255, 0, 0, 255, 255, 0, 0, 255, 255, 0, 0, 255,
  255, 0, 0, 255, 255, 0, 0, 255, 255, 0, 0, 255,
  255, 0, 0, 255, 255, 0, 0, 255, 255, 0, 0, 255,
  255, 0, 0, 255, 255, 0, 0, 255, 255, 0, 0, 255,
  255, 0, 0, 255, 255, 0, 0, 255, 255, 0, 0, 255,
  255, 0, 0, 255
]
Uint8Array(48) [
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0
]

The RGBA output is correct, but the RGB output is incorrect.