In HTML/CSS Part 3 Lesson 26, Collapsibles, I believe I have everything working now as Mosh shows. However, the Collapsible item is initially expanded. Shouldn’t it be collapsed when the page is first open, and then expanded when clicked?
I have seen an example for doing this sort of thing. The JavaScript is done differently to accomplish the initially-collapsed display. See Step 3 of How To Create a Responsive Top Navigation Menu.
Another option might be to have the extra class be “collapsed” instead of “collapsible–expanded”, and starting with the collapsed properties in the .css file.