DataTransfer.setDragImage() steps are unclear on how to size passed in <img> #13099

What is the issue with the HTML Standard?

Step 3 in DataTransfer.setDragImage() states:

3. If image is an img element, then set the drag data store bitmap to the element's image (at its natural size); otherwise, set the drag data store bitmap to an image generated from the given element (the exact mechanism for doing so is not currently specified).

For images that don't have a natural size, like dimensionless SVG images or an <img> with content: linear-gradient(...), its unclear what to do here.

For the most part, places where images need to be sized and displayed in some way like this should specify a concrete object size resolution algorithm to use.

Here though, I think it might make more sense to just defer to the engine so they can use whatever rules they use of drag-and-drop of images elsewhere. Or, if there is spec text about the normal (non-script) drag-and-drop of images, defer to that.

The one case that might be worth calling out is what to do about an <img> that is detached from the DOM, as that won't be handled in the non-script case. For that case, my suggestion would be to use the default sizing algorithm but allowing engines to set reasonable limits on maximum and minimum concrete sizes.