DomTerm (https://domterm.org) attempts to provide similar possibilities as TermKit. However, it starts with the position that it should also (and perhaps first) be a fully-functional modern mostly-xterm-compatible terminal emulator. On top of that we add rich html text, images, logical structure, "shell integration", and more.
It you like TermKit's pretty-printing of JSON, check out: https://domterm.org/saved/dt-prettynested.html
Try re-sizing the window width (to trigger smart re-breaking), and clicking the little triangles (to show/hide sub-trees of the output).
The actual pretty-printing is application-specific (for example the PrintNested java program at the top), but the output contains certain escape sequences to mark logical structure, optional indentation, and show/hide buttons.
Note this is basically a log of a terminal session from a few years ago, but saved as HTML, along with JavaScript to respond to the window-resize and show/hide actions.
