とりあえずマルチタッチな小さいコードcanvas2d版。

multitouch.html

<canvas width="512" height="512"></canvas><hr/>
<textarea id="LOG" cols="30" rows="10" value=""></textarea>
<script>"use strict";
let cx=document.querySelector("canvas").getContext("2d");
cx.canvas.addEventListener("touchstart",hander,false);
cx.canvas.addEventListener("touchmove",hander,false);
function hander(e){
	e.preventDefault();
	let touches=e.changedTouches;
	for (let i=0; i<touches.length; i++){
		drawRect(touches[i].pageX,touches[i].pageY, i);
	}
}
function drawRect(x,y,num){
	cx.fillStyle=["#f00","#0f0","#00f","#ff0","#0ff","#f0f",][num];
	cx.fillRect(x,y,16,16);	LOG.value+=`(${x},${y}),${cx.fillStyle}\n`;
}
</script>