@Terry44, it was just a basic example ideal for your mockup image workflow desires. But yes using a DIV allows for more manipulation and features via CSS and the current UI.
If it was to be an actual feature as previously hinted at …
Then it could be hidden from the user in the page tree, much like the “actions bar” etc., is on page items. Instead only accessible via it’s UI controls, and not directly by the user like a typical page item. I think this type of feature is also something that could help designer types better visualize as they set out to work with Pinegrow and use LIB , PROP , CSS , etc. Then as they add LIB items and change CSS properties they would begin to understand visually how those features work (by visual trial and error with their designs), in Pinegrow to accomplish their design and see visually the outcomes of those manipulations in comparison to their designs. After a while they would be less reliant on the underlying mockups and more in tune with the features and what needs are required.
Could be useful for some peeps. [ Feature Request - consideration @matjaz ]