insertHTML breaks DOM structure and formatting

Phenomenon

When using document.execCommand('insertHTML', ...) to insert HTML content into a contenteditable region, the DOM structure may be broken or reformatted unexpectedly. Nested elements may be flattened or reorganized.

Reproduction example

1. Create a contenteditable div.
2. Select a position within it.
3. Use document.execCommand('insertHTML', false, '<p><strong>Bold text</strong></p>').
4. Inspect the resulting DOM structure.

Observed behavior

• In Chrome on Windows, insertHTML may break the DOM structure.
• Nested elements may be flattened or reorganized.
• Formatting may be lost or changed unexpectedly.

Expected behavior

• insertHTML should preserve the HTML structure as provided.
• Nested elements should remain nested.
• Formatting should be maintained.