Full page editing in the shadow DOM

With allowRenderStylesFromHead enabled, the feature renders the <style> elements found in the data's <head> into the root the editable lives in: document.head in the light DOM, or the shadow root itself when the editor is attached to one. Loading new data must replace those styles, and destroying the editor must take them away.

The two data variants set different declarations for h2 and p, so a <style> left over from the previously loaded document shows up in the content, not only in the report. Their body rules are there to show the asymmetry between the roots: inside a shadow root such a rule matches nothing, while in the light DOM it repaints the whole page, this header included.

What to check:

• In both shadow modes, the styles apply to the editor content and the report counts them in the shadow root, with nothing stranded in document.head.
• Switching the data variant back and forth keeps the count at two – it must not grow – and the content picks up the new colors, with none of the previous variant's declarations still applying.
• Loading the variant with no head styles drops the count to zero.
• Destroy editor drops the count to zero as well, and the content of the destroyed editable stops being styled.
• Leaving source editing mode re-sets the data, so it goes through the same path as the Load data button.
• With Sanitize CSS on, the variant backgrounds stop applying; the rest of the CSS survives. With it off, the console logs css-full-page-provide-sanitize-function – expected, that is the feature warning about the missing sanitizer.
• The <script> in the data is preserved in the output but never runs: no alert.