Variable names in double curly braces don't evaluate in HTML export

For example:

[python cell]
my_var = 11

[markdown cell]
my_var evaluates to {{my_var}}

If the markdown cell is run in the jupyter notebook, it will say "my_var evaluates to 11". But if it's exported to html, it just says "my_var evaluates to {{my_var}}".

Start with the Python and markdown cell example from the issue, run it in a Jupyter notebook, and compare the rendered markdown with the HTML export. Trace the HTML export path to find where double-curly-brace variables are handled; done means the exported HTML renders "my_var evaluates to 11" rather than the literal placeholder.

Written by the indexing model from the issue text.