Who runs it
OpenAI. It names itself with the token ChatGPT-User.
What it does
It visits a page when a person asks ChatGPT or a custom GPT a question, and when someone uses an external application through GPT Actions. OpenAI says it is not used for crawling the web in an automatic fashion, and not used to decide whether content appears in search.
Renders JavaScript
Not yet observed. OpenAI doesn't say whether ChatGPT-User runs scripts, and this site hasn't recorded a verified visit yet.
How to verify it
Match the visiting address against OpenAI's published list. OpenAI documents no reverse DNS check.
robots.txt
OpenAI says that because these actions are initiated by a user, robots.txt rules may not apply. It points site owners to OAI-SearchBot for search opt-outs.
Questions
Does ChatGPT-User follow robots.txt?
OpenAI says robots.txt rules may not apply to it, because a person asked for the page.