At first, I thought I was unable to edit the code as well. I saw $html = ""; right before line 1. I put my cursor between the two quotes and tried to type. Nothing. Then I figured out what he’s doing. He’s showing that the $html variable is initially declared with a blank string as its content. Then on line 1 (the only area where you can type) you need to assign it a value. Like this: $html = "<div class="my-output">My shortcode output</div>"; I think it’s working, but confusing.
