JSFiddle - React, Tailwind, and code Playground

HTML

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
"http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
<svg xmlns="http://www.w3.org/2000/svg"
  xmlns:xlink="http://www.w3.org/1999/xlink" 
  width="300" height="300">
  <g id="firstGroup">
    <rect id="el" width="100" height="50" 
     x="40" y="20" fill="blue" />
    <text x="40" y="100">This is a basic SVG document!</text>
  </g>
</svg>
<div id='x'>X</div>

JavaScript

if(document.getElementById("el") instanceof SVGElement) {
    alert("It's SVG element");
}