JSFiddle - React, Tailwind, and code Playground
JavaScript
var doctype = document.documentElement.previousSibling;
// DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
document.body.innerHTML = "<b>Public ID:</b> " + doctype.publicId + "<br/>"
+ "<b>System ID:</b> " + doctype.systemId + "<br/><br/>"
+ "<b>Doctype string: </b>" + doctype.nodeValue;