We have an honest implementation of HTML/CSS/JS on our site. The code you or your students write is uploaded to a server and then served as it would be in a real world web server deployment.

This allows us to support things such as linking between multiple HTML pages and the ability to include multiple JS, CSS and -- very soon -- arbitrary assets.

To link HTML files, just add a file, name it (with an .html extension), and then include a link to it in the following format: <a href="file.html">link</a>.

Here is a live example: repl.it/DjWo/1.