Looks like Shiki: https://github.com/shikijs/shiki https://github.com/shikijs/shiki If you "inspect" the elements with the mouseovers, you can see they statica

Looks like Shiki: https://github.com/shikijs/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.