Verify the processing boundary
Look for a clear local-processing statement and inspect network activity when the material is particularly sensitive.
Remove secrets before comparison
A local tool reduces exposure but does not make the surrounding device safe. Redact passwords, tokens, private keys, and personal identifiers when they are not needed.
Avoid persistent history
Use an incognito or dedicated profile on shared devices and close the tab after saving the required result.
Treat exports as sensitive
A diff can reveal deleted content as well as additions. Store and share it under the same controls as the source documents.
Check the entire data path
Local calculation only protects the comparison step. Clipboard managers, browser extensions, operating-system indexing, synced download folders, screen sharing, and device backups can still retain the material. Use a trusted browser profile, disable extensions that can read pages when the stakes justify it, and avoid pasting secrets that are irrelevant to the change review. For regulated or contractually restricted material, follow the applicable organizational process rather than assuming that a public browser tool is approved.
Verify and clean up deliberately
A practical check is to open developer tools before pasting a harmless sample, clear the network log, run the comparison, and confirm that no request contains the sample. This site has no application upload endpoint or analytics SDK, but Cloudflare still receives ordinary requests needed to serve the static page. After the real comparison, download only the evidence you need, close the tab, clear any clipboard history under your control, and handle the diff with the same access rules as both source versions.