Any idea how to achieve this by CSS or script in nodebb ACP?

link :https://www.tuhh.de/MathJax/test/sample-dynamic.html

you can easily check the source code of it, here I copy it in below

<!DOCTYPE html> <html> <head> <title>MathJax Dynamic Math Test Page</title> <!-- Copyright (c) 2010-2015 The MathJax Consortium --> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <meta http-equiv="X-UA-Compatible" content="IE=edge" /> <script type="text/javascript" src="../MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script> <style> input {margin-top: .7em} .output { border: 1px solid black; padding: 1em; width: auto; position: absolute; top: 0; left: 2em; min-width: 20em; } .box {position: relative} </style> </head> <body> <script> // // Use a closure to hide the local variables from the // global namespace // (function () { var QUEUE = MathJax.Hub.queue; // shorthand for the queue var math = null, box = null; // the element jax for the math output, and the box it's in // // Hide and show the box (so it doesn't flicker as much) // var HIDEBOX = function () {box.style.visibility = "hidden"} var SHOWBOX = function () {box.style.visibility = "visible"} // // Get the element jax when MathJax has produced it. // QUEUE.Push(function () { math = MathJax.Hub.getAllJax("MathOutput")[0]; box = document.getElementById("box"); SHOWBOX(); // box is initially hidden so the braces don't show }); // // The onchange event handler that typesets the math entered // by the user. Hide the box, then typeset, then show it again // so we don't see a flash as the math is cleared and replaced. // window.UpdateMath = function (TeX) { QUEUE.Push(HIDEBOX,["Text",math,"\\displaystyle{"+TeX+"}"],SHOWBOX); } })(); </script> <p> Type some \(\rm\TeX\) code and press RETURN:<br /> <input id="MathInput" size="80" onchange="UpdateMath(this.value)" /> </p> <p>You typed:</p> <div class="box" id="box" style="visibility:hidden"> <div id="MathOutput" class="output">$${}$$</div> </div> <script> // // IE doesn't fire onchange events for RETURN, so // use onkeypress to do a blur (and refocus) to // force the onchange to occur // if (MathJax.Hub.Browser.isMSIE) { MathInput.onkeypress = function () { if (window.event && window.event.keyCode === 13) {this.blur(); this.focus()} } } </script> </body> </html>