Looks like Shiki: https://github.com/shikijs/shiki
If you "inspect" the elements with the mouseovers, you can see they statically compiled the TS definition into the HTML.
I guess it probably uses the same APIs to read the types VSCode does, given what they state the purpose of Shiki is.
Right? I'm 99% sure this thing uses https://github.com/shikijs/shiki
The tool on GitHub: https://github.com/shikijs/shiki You can import any of the grammars or themes from VSCode that are not currently in the project.
I found this because I wanted to reuse the same grammar for CUE on GitHub for my statically generated website. I was using Chroma previously.