JSFiddle - React, Tailwind, and code Playground

HTML

<p>
<canvas id="cv" width="15" height="15"></canvas>
</p>
<p>
Found colors:
<div id="debug"></div>

JavaScript

var cv = document.getElementById("cv");
var ctx = cv.getContext("2d");
ctx.fillStyle = "#ffffff";
ctx.strokeStyle = "#000000";
ctx.lineWidth = 3;
ctx.lineCap = "round";
ctx.lineJoin = "bevel";
ctx.fillRect(0, 0, cv.width, cv.height);
ctx.moveTo(5, 5);
ctx.lineTo(10, 10);
ctx.stroke();
var imgdata = ctx.getImageData(0, 0, cv.width, cv.height);
for (var y = 0; y < cv.height; y ++)
{
  for (var x = 0; x < cv.width; x ++)
  {
    var idx = 4 * (y * cv.width + x);
    var c = imgdata.data[idx]<<16 | imgdata.data[idx+1]<<8 | imgdata.data[idx+2];
    if (c != 0xffffff)
    {
      document.getElementById("debug").innerHTML += x+"x, "+y+"y: "+c.toString(16)+"<br/>";
    }
  }
}