There is a problem with this comparison. The agent had access to open-source browsers in its training set. So you'd need to compare the cost of creating an equivalent browser for a developer who has access to them, too. If all you need is standard browser functionality, you just use an existing browser. If you want to change some features or parts of the implementation, you fork it. A new browser written from scratch would be valuable if it had a novel implementation that resulted in a faster/more secure/robust/memory efficient or simply easier-to-use browser. So even if this had implemented the standard correctly, it wouldn't be worth more than the time it takes a developer to fork Chromium and change its name. Don't get me wrong, it's impressive, but not as impressive after you think that an LLM that regurgitates verbatim the code of Chromium when tasked to build a browser would have effectively succeeded at the task.

EDIT: About the rendering speed. It doesn't really make sense to compare it with a fully functioning browser, as you could potentially drop features or make bogus optimisations to go faster.