<div class="box" style="--v:calc(100% - 4em)"> </div>
<div class="box" style="--v:calc(100% + 10% - 50% - 4em + 2px)"> </div>
<div class="box" style="--v:calc(100% + 10% - 50% - 4em + 2px)"> </div>
<div class="box" style="--v:calc(80% - 5ch)"> </div>
<div class="box" style="--v:calc(-50px + 80%)"> </div>



<script>
if (CSS.paintWorklet) {              CSS.paintWorklet.addModule('/t_afif/pen/mdwRVBr.js');
}  else {
  alert("Your browser cannot run this demo. Consider Chrome or Edge instead")
}
</script>

@property --border{
  syntax: '<length>';
  inherits: true;
  initial-value: 0;
}
@property --v{
  syntax: '<length-percentage>';
  inherits: true;
  initial-value: 0px;
}

.box {
  --path:0 0,var(--v,0) 0,100% 40px,100% 100%,0 100%;
  --border:5px;
  margin:5px 20px;
  width:200px;
  height:100px;
  cursor:pointer;
  background:red;
  display:inline-block;
  clip-path:polygon(var(--path));
  -webkit-mask:paint(polygon-border)
}

body {
  background:pink;
}

registerPaint('polygon-border', class  {

  static get inputProperties() {
    return [
      '--path',
      '--border',
      '--dash'
    ]
  }
  
  paint(ctx, size, properties) {

    const points = properties.get('--path').toString().split(',');
    const b = parseFloat(properties.get('--border').value);
    const d = properties.get('--dash').toString().split(',');
    const w = size.width;
    const h = size.height;
    console.log(points);
    
    const cc = function(x,y) {
      var fx=0,fy=0;
      if (x.indexOf('calc') > -1) {
        var tmp = x.replace('calc(','').replace(')','');
        if (tmp.indexOf('+') > -1) {
          tmp = tmp.split('+');
          fx = (parseFloat(tmp[0])/100)*w + parseFloat(tmp[1]);
        } else {
          tmp = tmp.split('-');
          fx = (parseFloat(tmp[0])/100)*w - parseFloat(tmp[1]);
        }
      } else if (x.indexOf('%') > -1) {
        fx = (parseFloat(x)/100)*w;
      } else if(x.indexOf('px') > -1) {
        fx = parseFloat(x);
      }
      
      if (y.indexOf('calc') > -1) {
        var tmp = y.replace('calc(','').replace(')','');
        if (tmp.indexOf('+') > -1) {
          tmp = tmp.split('+');
          fy = (parseFloat(tmp[0])/100)*h + parseFloat(tmp[1]);
        } else {
          tmp = tmp.split('-');
          fy = (parseFloat(tmp[0])/100)*h - parseFloat(tmp[1]);
        }
      } else if (y.indexOf('%') > -1) {
        fy = (parseFloat(y)/100)*h;
      } else if(y.indexOf('px') > -1) {
        fy = parseFloat(y);
      }
      return [fx,fy];
    }
    
    var p = points[0].trim().split(/(?!\(.*)\s(?![^(]*?\))/g);
    p = cc(p[0],p[1]);
    ctx.beginPath();
    ctx.setLineDash(d);
    ctx.moveTo(p[0],p[1]);
    
    for (var i = 1; i < points.length; i++) {
      p = points[i].trim().split(/(?!\(.*)\s(?![^(]*?\))/g);
      p = cc(p[0],p[1]);
      ctx.lineTo(p[0],p[1]);
    }
    ctx.closePath();
    ctx.lineWidth = 2*b;
    ctx.strokeStyle = '#000';
    ctx.stroke();
  }

})