JavaScript

Try to learn: Regular Expression HTML CSS and JavaScript

Seemof Nick Niemeir's site on JavaScript

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One can simply insert javascript into html:

<script> </script>

More commonly one creates a .js file and calls it from the HTML.

<script src="assets/scripts/app.js"></script>

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A programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for web page behavior, often incorporating third-party libraries. All major web browsers have a dedicated JavaScript engine to execute the code on users' devices.

https://en.m.wikipedia.org/wiki/JavaScript HEIGHT 400 Wikipedia