This is an umbrella bug for all bugs which are required to implement SVG 2. The specs page is: https://www.w3.org/TR/SVG2/.
<rdar://problem/90110236>
This should also include allowing ‘<use>’ to reference an external document's root element by omitting the fragment. https://svgwg.org/svg2-draft/single-page.html#struct-UseElement I keep each of my icons in a separate SVG file. Needing to add an ID to the <svg> in each of my icon files shouldn't be necessary. This way I can refer to icons in the same way regardless of whether I am using them as a background-image or as with <use> in HTML.