Can you give me an example of what you're trying to replicate?
Codepen splits code into: HTML / CSS / JS
HTML can be inserted within the inner HTML of an element. You would create a rectangle, select it, and click edit > edit inner html, and paste that code.
For CSS, you would put this in the 'head'. You can edit the contents of the
<head>…</head> of your exported .html file by clicking on 'Edit HTML Head' in the Document Inspector. You may need to uncheck 'Protect from External Styles' in the document inspector depending on what you're doing.
For JS, this is trickier to answer, and depends on when it should run. Most can run as an 'On Scene Load' action.