Clipboard API read/write works inconsistently with contenteditable

Phenomenon

When using the Clipboard API (navigator.clipboard.readText/writeText) with contenteditable elements, the behavior may be inconsistent. Reading clipboard content may not work during paste events, and writing to clipboard may not preserve formatting.

Reproduction example

1. Create a contenteditable div.
2. Listen for paste events.
3. Try to read clipboard content using Clipboard API.
4. Try to write formatted content to clipboard.
5. Observe any errors or inconsistencies.

Observed behavior

• In Firefox on Windows, Clipboard API may not work correctly with contenteditable.
• Reading clipboard during paste may require user gesture.
• Writing formatted content may not preserve HTML.
• Permissions may be required inconsistently.

Expected behavior

• Clipboard API should work seamlessly with contenteditable.
• Reading should work during paste events.
• Writing should preserve formatting when appropriate.