Syntax coloring doesn't work

Hi, I'm the maintainer of [domical](https://github.com/louisabraham/domical)
I just updated my plugin to work with the latest version of iodide.
I'm trying to get the syntax coloring to work but I have no idea why it fails: https://alpha.iodide.io/notebooks/1627/

I tested in Firefox and Safari.

Start by opening the linked domical notebook and reproducing the syntax-coloring failure in Firefox and Safari. Trace the notebook's interaction with the latest iodide version; done means syntax coloring works for the updated plugin without regressing the reported environments.

Written by the indexing model from the issue text.