I often see a demo for some library or other, and I want to try to make a small change. So I have to download the entire page with all dependencies and then make the change there. I'd like to be able to make the change when I view the source code in the browser.
Tampermonkey is not really a High-Level HTML editor, though.
For instance this script: https://greasyfork.org/en/scripts/536-imdb-standard-deviation/code
Arithmetic mean = 7.5 Median = 8
: Arithmetic mean = 7.5 Median = 8 Standard Deviation = 2.02
on the IMDB Ratings page of 'Black Mirror' movie