I will investigate why the HTML is not displaying.
Since you say you’re working on progress bars, there’s a limitation you should know about. Display messages (e.g. matplotlib plots, or this HTML stuff) only get sent to the client at the end of cell execution. If you want to have things appearing in the client during cell execution, you’ll have to print to stdout.
If updating an HTML display is very important to you, then I could implement a hacky workaround where stdouts containing a special magic string get displayed as HTML on the client.
(Details about this limitation: In order to send display messages during cell execution, we need to send ZMQ messages from the Swift process to Jupyter. To do this, we need to run the ZMQ library in the Swift process. The ZMQ library starts up its own threadpool for sending messages. LLDB (which has control over the Swift process) likes to halt these threads in sometimes, which prevents ZMQ from sending messages. So to remove this limitation, I’ll have to figure out why LLDB likes to halt the threads, and modify LLDB, and I think this might be quite difficult.)