- The preview re-renders 300 ms after the last WYSIWYG edit, Source input or external value, never while off or after detach.
- YouTube placeholders show as img.youtube.com thumbnail cards with no player; the preview copy drops active elements and on* attributes, and its stylesheet uses colours resolved from the host's --c-* values.
- A per-element Preview toggle (aria-pressed, off by default) splits the editor 50/50 with an auto-fit grid that wraps the pane below on narrow widths.
- The pane is an iframe with an empty sandbox, content set only through srcdoc, created on first use; it reads only the value getter, so previewing never edits or emits summer-change.
