Go to an artifact.

Start to enter a follow up comment in text format. Example:

I suggest the following approach:

* a
* b

Then, switch the format to HTML => The text should not be condensed on one line. Basic formatting should be kept.

Now start to enter a follow up comment in HTML format (add bold, italic or list elements).
Then switch the format to text => The textarea should not contain HTML tag soup. Basic formatting should be kept.

tl;dr: I should be able to switch back and forth between text and HTML format without losing too much in the formatting.