Tooltips and fixed dropdowns pinned to the position of page elements are already fairly trivial to do... so are modals... so are dialog boxes... so are every other thing that needs to "Pop Up" on your webpage. I predict a very fast death spiral for this tag, in which 99% of the uses will be obnoxious.
You know how, uh, ad blockers used to be called "popup blockers"?
There's an order, a z-indexing that creates a syntax and a hierarchy in everything you build. Sometimes you have to jump it with absolute positioning. Which already exists for this exact purpose.
CSS itself has suffered from this same line of thinking, like, "what if something has to override a class and override a style, should we call it !important? Yeah!"
On a large complex single page webapp, it can be very non-trivial to have a popup/modal over everything else in the page. That’s why things like React Portals[0] exist, as well as many, many libraries that provide similar functionality. z-index is not an absolute number and depends on your stacking context.
[0] https://react.dev/reference/react-dom/createPortal
How is what that's doing any different from figuring out element A's desired position in the coordinate space of the body, or in the local coordinate space of element B, and then appending it to one or the other in an absolute positioned wrapper? I've written very large single page webapps that have dropdown menus with sub-menus that are almost entirely CSS with just small amounts of vanilla JS to adjust their screen positions. Sometimes a really lightweight library is useful, but requiring the whole React framework seems like total overkill. On the other hand, adding a new HTML tag for the purpose of popup modals seems unnecessary.
You know how, uh, ad blockers used to be called "popup blockers"?
There's an order, a z-indexing that creates a syntax and a hierarchy in everything you build. Sometimes you have to jump it with absolute positioning. Which already exists for this exact purpose.
CSS itself has suffered from this same line of thinking, like, "what if something has to override a class and override a style, should we call it !important? Yeah!"