You can use https://vscode.dev online without logging in (it's a bit nerfed compared to installing the desktop app though), then validate using a site like https://validator.w3.org (this is limited to HTML4 though, might want a different one if you're using the latest HTML features). Good luck OP More on reddit.com

hello More on reddit.com

You could use a code playground as another user said, but the more beneficial approach would probably be with a code editor and an html file. I say more beneficial because writing code like this will let you build your own projects in a style more similar to writing code that would go into production. Simply download a code editor (Visual Studio Code is great), create a JavaScript file (.js), an html file (.html), and connect them. In the header of your html file add and replace app with whatever you name your JS file. The script tag will "connect" your html and js file, so whenever you save your js file and refresh it will be reflected in the browser. You only open your html file in the browser, if you open your js file you'll just see the code you've written and nothing will be executed. To open the html file if you're on Mac go to the browser and open Finder with command + O. If you're on windows it's something like control + O I think. Then just find the html file and open it. More on reddit.com

You can use a vscode addon called live server. Then right click the html file in vscode and click the option with live server in the name. It auto opens it in your webbrowser. It even auto reloads it when you change and save a file. More on reddit.com