A standard HTML/CSS/JS editor, which also contains a hidden script and an import script for the web-worker:

A challenge-style editor with two run buttons, one of which runs editable code (example tests), and the other of which runs hidden code (submission tests):

A demonstration of pre- and post-HTML snippets and hidden CSS styling, a disabled console, and the ability to modify editor content programmatically:

A demonstration of HTML/CSS testing (and also how auto-run and auto-preview can work together):