How does GitHub detect whether the browser has color emoji support?

Question

When visiting GitHub using a browser without color emoji support, it loads emoji glyphs as image files.

But when visiting GitHub using a browser that does have color emoji support, it lets the browser render the glyphs normally instead.

How can GitHub know whether the browser supports color emoji or not?

Answer 1

I do that like this :

var canvas = document.createElement("canvas"),
    ctx = canvas.getContext("2d");

canvas.style="height:100px;width:100px;position:absolute;top:60%;background:white";
document.body.appendChild(canvas);
ctx.fillStyle ="#FFF";
ctx.font="100px Arial";
ctx.fillText('😡',100,100);
var d = ctx.getImageData(50, 50, 1, 1).data;
if (d[0] + d[1] + d[2] === 0) {
    //need twemoji or something else
} else {
    //nothing to do   
}