One self-contained HTML file plus your session transcript (file or link). Resubmitting inside the window replaces your earlier entry — the last one counts. The server clock is the referee.
If you link instead of uploading, make sure the link opens for someone who is not you. Private workspace and cowork session links only open for their owner, so nobody can read them and your session cannot be quoted if you win. Test it in a private browser window first, or use your tool's export and upload the file above.
Transcripts of the top entries are read before results publish, and parts of the winning one get quoted in the round write-up. Exports sometimes carry things you did not mean to publish, file paths with your name in them, account ids, pasted keys. Worth a glance before you upload. Nothing is ever published without the prompts being checked first.