zig Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  height: 2000px;
  background: #f1f1f1;
}
#mySVG {
  position: fixed;
  top: 15%;
  width: 400px;
  height: 210px;
  margin-left:-50px;
}
</style>
</head>
<body>
<h2>Scroll down this window to draw a triangle.</h2>
<p>Scroll back up to reverse the drawing.</p>
<svg id="mySVG">
  <path fill="none" stroke="red" stroke-width="3" id="triangle" d="M150 0 L75 200 L225 200 Z" />
  Sorry, your browser does not support inline SVG.
</svg>
<script>
  // Get the id of the <path> element and the length of <path>
  var triangle = document.getElementById("triangle");
  var length = triangle.getTotalLength();
  // The start position of the drawing
  triangle.style.strokeDasharray = length;
  // Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
  triangle.style.strokeDashoffset = length;
  // Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
  window.addEventListener("scroll", myFunction);
  function myFunction() {
    var scrollpercent = (document.body.scrollTop + document.documentElement.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);
    var draw = length * scrollpercent;
    // Reverse the drawing (when scrolling upwards)
    triangle.style.strokeDashoffset = length - draw;
  }
</script>
</body>

</html>