Edit in JSFiddle

var arc = function (ctx, a, b, o, r) {
    var t1 = Math.atan2(a[1] - o[1], a[0] - o[0]);
    var t2 = Math.atan2(b[1] - o[1], b[0] - o[0]);

    ctx.beginPath();
    ctx.arc(o[0], o[1], r, t1, t2);
    ctx.stroke();
};

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgb(255,0,0)";
ctx.strokeStyle = "rgb(0,150,0)";
ctx.lineWidth = 4;

var STEPS = 60;
var PAUSE = 30;
var PAUSE_PT = STEPS * 5 / 12;
var R = 60;

var CNTPT = [100, 80];
var ancpt = [100, 20];
var varpt = [160, 80];

var animate = function (i) {
    ctx.clearRect(0, 0, 200, 200);

    var I;
    if (i < PAUSE_PT) I = i;
    else if (i < PAUSE_PT + PAUSE) I = PAUSE_PT;
    else I = i - PAUSE;

    var t = Math.PI * 2 * (I / STEPS - 1/4);
    var cntpt = null;
    var r = null;

    varpt = [CNTPT[0] + Math.cos(t) * R, CNTPT[1] + Math.sin(t) * R];
    if (I > PAUSE_PT) {
        var q = Math.sqrt(Math.pow(ancpt[0] - varpt[0], 2) +
                          Math.pow(ancpt[1] - varpt[1], 2));
        r = (q / 2) / Math.sin(Math.PI * 2 / 6);

        var dir = [(varpt[1] - ancpt[1]) / q, (ancpt[0] - varpt[0]) / q];
        var avg = [(ancpt[0] + varpt[0]) / 2, (ancpt[1] + varpt[1]) / 2];
        cntpt = [avg[0] - Math.sqrt(r * r - q * q / 4) * dir[0],
        avg[1] - Math.sqrt(r * r - q * q / 4) * dir[1]];
    }

    arc(ctx, ancpt, varpt, cntpt == null ? CNTPT : cntpt, r == null ? R : r);
    setTimeout(function () {
        animate((i + 1) % (STEPS + PAUSE))
    }, 15);
}

animate(0);

<canvas id="canvas" width=200 height=200></canvas>

canvas {
    border-style: solid;
    border-color: black;
    border-width: 1px;
}