Yes, the rendered output, using the browsers copy tools, with alternatives available as Browser addons.
When you select some nicely formatted content found on any website, not just tiddlywiki the “formatted text is often copied to the clipboard”, then when you paste it somewhere it is up to the application you are pasting it into to deal with the formatting. Some only paste plain text, others will convert the formatting to its own WYSIWYG formatting.
- GMAIL accepts such formatted content and includes it in the email with this formatting, and I believe it does this as HTML.