Thought I had completed the markdown previewer, however I’m failing half the tests! Functionally it seems fine, and when I use the dev tools it is indeed inserting HTML in the #preview element section based on what I type in the #editor element but when I included the testing script locally to test it this is what I’m getting: https://imgur.com/oXOYVuo
It is very difficult to debug with a picture and a live site without the test-suite. I suggest you provide a link to your project on a platform like Repl.it, CodeSandbox, or Glitch.
It should be as simple as forking the repo from those sites.