A line dump is not a document review
Pairing leftover lines by position treats a heading that moved down as a deletion plus an unrelated insertion. That hides the real story: the section is still there, and a new one may have arrived later.
MD Viewer's diff checker walks shared lines with a shortest-edit comparison, then names heading changes from the same Markdown engine as preview. Git is not required. The files never leave this browser.
Open original and revised
Treat the left pane as the older document and the right pane as the candidate. Open or paste each side, or drop two .md files onto the page. The selected Markdown profile controls how headings are parsed, so keep it aligned with the profile you use in the editor.
- Use GitHub Extended for README files that include tables, task lists, or footnotes.
- Switch to CommonMark when you need the most portable outline.
- Line endings that differ are reported as a note instead of rewriting every line.
Read the outline before the red lines
Start on Outline when a spec grew a chapter or a heading changed level. Then use Source for the line walk, and Preview to see both documents with the same renderer as the editor.
The source view can sit side by side or in a unified patch. Hunk headers include the nearest heading so a change under "Before release" does not look like a change under "Support".
Keep a patch if the review has to travel
Download the unified diff when someone else needs the same comparison, or when you want a file next to the two Markdown sources. The patch is generated in this browser from the current panes.
Do not treat the patch as a merge. It is a record of differences. If you need one document assembled from several chapters, use the merger instead of concatenating the revised file over the original.