SOURCE

    var refreshDuration = 5000;
    var refreshTimeout;
    var numPointsX;
    var numPointsY;
    var unitWidth;
    var unitHeight;
    var points;

    function onLoad()
    {
        var svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
        svg.setAttribute('width',window.innerWidth);
        svg.setAttribute('height',window.innerHeight);
        document.querySelector('#bg').appendChild(svg);

        var unitSize = (window.innerWidth+window.innerHeight)/20;
        numPointsX = Math.ceil(window.innerWidth/unitSize)+1;
        numPointsY = Math.ceil(window.innerHeight/unitSize)+1;
        unitWidth = Math.ceil(window.innerWidth/(numPointsX-1));
        unitHeight = Math.ceil(window.innerHeight/(numPointsY-1));

        points = [];

        for(var y = 0; y < numPointsY; y++) {
            for(var x = 0; x < numPointsX; x++) {
                points.push({x:unitWidth*x, y:unitHeight*y, originX:unitWidth*x, originY:unitHeight*y});
            }
        }

        randomize();

        for(var i = 0; i < points.length; i++) {
            if(points[i].originX != unitWidth*(numPointsX-1) && points[i].originY != unitHeight*(numPointsY-1)) {
                var topLeftX = points[i].x;
                var topLeftY = points[i].y;
                var topRightX = points[i+1].x;
                var topRightY = points[i+1].y;
                var bottomLeftX = points[i+numPointsX].x;
                var bottomLeftY = points[i+numPointsX].y;
                var bottomRightX = points[i+numPointsX+1].x;
                var bottomRightY = points[i+numPointsX+1].y;

                var rando = Math.floor(Math.random()*2);

                for(var n = 0; n < 2; n++) {
                    var polygon = document.createElementNS(svg.namespaceURI, 'polygon');

                    if(rando==0) {
                        if(n==0) {
                            polygon.point1 = i;
                            polygon.point2 = i+numPointsX;
                            polygon.point3 = i+numPointsX+1;
                            polygon.setAttribute('points',topLeftX+','+topLeftY+' '+bottomLeftX+','+bottomLeftY+' '+bottomRightX+','+bottomRightY);
                        } else if(n==1) {
                            polygon.point1 = i;
                            polygon.point2 = i+1;
                            polygon.point3 = i+numPointsX+1;
                            polygon.setAttribute('points',topLeftX+','+topLeftY+' '+topRightX+','+topRightY+' '+bottomRightX+','+bottomRightY);
                        }
                    } else if(rando==1) {
                        if(n==0) {
                            polygon.point1 = i;
                            polygon.point2 = i+numPointsX;
                            polygon.point3 = i+1;
                            polygon.setAttribute('points',topLeftX+','+topLeftY+' '+bottomLeftX+','+bottomLeftY+' '+topRightX+','+topRightY);
                        } else if(n==1) {
                            polygon.point1 = i+numPointsX;
                            polygon.point2 = i+1;
                            polygon.point3 = i+numPointsX+1;
                            polygon.setAttribute('points',bottomLeftX+','+bottomLeftY+' '+topRightX+','+topRightY+' '+bottomRightX+','+bottomRightY);
                        }
                    }
                    polygon.setAttribute('fill','rgba(0,0,0,'+(Math.random()/3)+')');
                    var animate = document.createElementNS('http://www.w3.org/2000/svg','animate');
                    animate.setAttribute('fill','freeze');
                    animate.setAttribute('attributeName','points');
                    animate.setAttribute('dur',refreshDuration+'ms');
                    animate.setAttribute('calcMode','linear');
                    polygon.appendChild(animate);
                    svg.appendChild(polygon);
                }
            }
        }

        refresh();

    }

    function randomize() {
        for(var i = 0; i < points.length; i++) {
            if(points[i].originX != 0 && points[i].originX != unitWidth*(numPointsX-1)) {
                points[i].x = points[i].originX + Math.random()*unitWidth-unitWidth/2;
            }
            if(points[i].originY != 0 && points[i].originY != unitHeight*(numPointsY-1)) {
                points[i].y = points[i].originY + Math.random()*unitHeight-unitHeight/2;
            }
        }
    }

    function refresh() {
        randomize();
        for(var i = 0; i < document.querySelector('#bg svg').childNodes.length; i++) {
            var polygon = document.querySelector('#bg svg').childNodes[i];
            var animate = polygon.childNodes[0];
            if(animate.getAttribute('to')) {
                animate.setAttribute('from',animate.getAttribute('to'));
            }
            animate.setAttribute('to',points[polygon.point1].x+','+points[polygon.point1].y+' '+points[polygon.point2].x+','+points[polygon.point2].y+' '+points[polygon.point3].x+','+points[polygon.point3].y);
            animate.beginElement();
        }
        refreshTimeout = setTimeout(function() {refresh();}, refreshDuration);
    }

    function onResize() {
        document.querySelector('#bg svg').remove();
        clearTimeout(refreshTimeout);
        onLoad();
    }

    window.onload = onLoad;
    window.onresize = onResize;

<div id="bg"></div>

html {
  background: radial-gradient(ellipse at center, #ef0e39 0%, black 100%);
  overflow: hidden;
}