1. Go to https://apple.com/
2. Open Web Inspector
3. Go to the sources Tab in web inspector and select a file
4. Click on the two files icon for creating a local override.
5. Select from the menu "Create Response Local Override"
6. Modify a value or a couple of lines. 

Expected: have a visual indicator of the differences in between the original file and the local override version. 
It could be a word-diff, but it could also be a simple red bar near the line number or in a scrollbar colon showing the visual map of the source.