The following bindings can be converted to custom elements by prepending/appending elements when CE connects.
They would need the
basecontrol mixin to be landed in bug 1518932.
Specifically for tabbrowser-tab we could move the complexity to its only user if applicable.
Watch out for performance issue with a lot of tabs though.
Has <content> + <children>, but it’s only prepending and appending. Used lightly (devtools and maybe one other place).
Coupled with tab. Could be used as a tab with [is].
Look into performance + sessionrestore with a lot of tabs