[CSS Anchored Positioning] Selecting the anchor origin (feedback/questions)

In the explainer, it seems like the only way to select the origin of the anchor is using the anchor attribute of a popup element, which would reference another element's ID. I find this to be a bit cumbersome, as it means the following:

• anchored positioning is limited to the popup element
• The document needs to manage IDs of all the elements that might have a popup anchored to them. In documents with a lot of tooltips and popup-like functionalities, this can get quite hairy.

I would propose to have an additional CSS property, perhaps anchor-origin like in one of the spec alternatives (or anchor-target?), which would take either an ID or a selector() function (like in Scroll Linked Animations. For popup, it would be automatically be linked to the popup's anchor attribute, but would be available for any fixed-position element, and would be what defines whether a fixed-position element is anchored to an element or to the viewport.