Well, there are lots of resource which can help you to develop editor using CSS / HTMl and JS.
Here is the code reference.
<!DOCTYPE html>
<html>
<head>
<title>Simple Text Editor</title>
<style>
body {
font-family: Arial, sans-serif;
}
textarea {
width: 100%;
height: 300px;
padding: 10px;
border: 1px solid #ccc;
border-radius: 5px;
}
</style>
</head>
<body>
<h1>Simple Text Editor</h1>
<textarea id="editor"></textarea>
<script>
const editor = document.getElementById('editor');
Thanks