Open issues are kept track of here: https://wiki.whatwg.org/wiki/Custom_Elements
This has come up before, but it came up again at the Extensible Web Summit so raising hopefully for the last time. The DOM has insert/remove primitives for nodes. Custom Elements uses "insertion into a document" and "removal from a document" which are quite a bit different and higher level. Why should we have different primitives here? -- https://annevankesteren.nl/