Before-and-after Markdown review: compare rendered sections

To review a Markdown edit, preview both versions with the same renderer and viewport. Compare heading hierarchy, list nesting, code formatting, link destinations, and paragraph spacing. Changes to blank lines or indentation can alter large sections even when only a few characters were edited. Keep the source diff alongside the rendered comparison so reviewers can connect a visible change to its cause. If the destination sanitizes HTML, test there rather than relying solely on a local preview.