chromium/src/third_party/blink/web_tests/http/tests/lazyload/placeholder.js

// Returns if the image is complete and fully loaded as a non-placeholder image.
function is_image_fully_loaded(image) {
  if (!image.complete) {
    return false;
  }

  let canvas = document.createElement('canvas');
  canvas.width = canvas.height = 1;
  let canvasContext = canvas.getContext("2d");
  canvasContext.drawImage(image, 0, 0);
  let data = canvasContext.getImageData(0, 0, canvas.width, canvas.height).data;

  // Fully loaded image should not be a placeholder which is drawn as a
  // translucent gray rectangle in placeholder_image.cc
  return data[0] != 0xd9 || data[1] != 0xd9 || data[2] != 0xd9;
}