In order to directly edit the HTML on LibGuides, navigate to the "edit" button in the drop-down on the bottom right of a block, then click the "Source" button. When you're done, save it as usual. This method is more useful than choosing the "</>html" option in the initial drop-down. This way, you can click the "Source" button to toggle between the HTML view and the website view. This allows you to see how your edits will look in real-time as you edit them.
See below for a visual step-by-step:
