HTML spec
- Add the `fetchpriority` attribute to the `img`, `link` and `script` elements.
- Add the `plaintext-only` value to the `contenteditable` global attribute.
- Add the `search` element.
- Replace the `popoverhidetarget`, `popovershowtarget` and `popovertoggletarget`
attributes of the `button` and `input` elements with `popovertarget` and
`popovertargetaction`.