The name of the element to mimic an article on a web browser in draw.io

Currently it isn't possible for this shape. In the next release we'll add address and tab parameters. Generally you can do this for most mockup shapes: select a shape, go to format->style and in this window you can change optional parameters. The dialog is a temporary solution and there will be a better method soon.

From what I can tell there's no setting for making something collapsible, nor can you add it as code (not in the code for the object itself and not in the code for the whole page). I'm not an expert though.

The only thing I can think of, and you've probably thought of this hack, is to add a container (no separator and no fill) for each element that you want to be able to collapse, then put your element inside it. Make the line color of the container "none". Size the element and position it in such a way that the little collapsing UI box is in the position you want relative to your element.

Just be sure that when you move that thing around you're moving the container, not just the element.