katex.render doesn’t render equation – Blancer.com Tutorials and projects

I am new for html, javascript, and katex. I wonder if you could tell me why the following simple html does not show the equation.

<!DOCTYPE html>
<html>

<head>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.css"
    integrity="sha384-yW1frltt4cd2K6QCmOMx4M+ylOmyrGb+NDouZ1f5VL2f4NN7uBu5PQsmUSgWib1W" crossorigin="anonymous">
<script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.js"
    integrity="sha384-ZfxPSr1+NHNHrv9KI48JF/1RkvdoxKWJPuYyfxE+iexlsGw8tyQR4OuNVNUuzwLN"
    crossorigin="anonymous"></script>
<title>My app</title>
</head>

<body>
<span id="idequ">equation</span>
<script>
    katex.render("f(a,b,c) = (a^2+b^2+c^2)^3", document.getElementById("idequ"));
</script>
</body>