This gist simply lists all elements that can't be extended on "the platform" if not through the Custom Elements builtin extends feature.
This list does not focus on the "why would you?" rather on the "why can't you?" (on Safari) question out there, using the Permitted Parent section out of MDN Element Reference.
- html
- base
- head
- link
- meta
- style - any parent but no way to have it without disrupting the layout
- title
- body
- main - has to be hierarchly correct
- dd
- dt
- br - any parent but no way to have it without disrupting the layout
- hr - any parent but no way to have it without disrupting the layout
- figcaption
- li
- rp
- rt
- area
- track
- source
- script - any parent but no way to have it without disrupting the layout
- caption
- col
- colgroup
- tbody
- td
- tfoot
- th
- thead
- tr
- legend
- optgroup
- option
- summary