You can select an HTML element to view where its content came from. The screenshot shows that the string "Hello" originally came from localStorage.
Other possible string sources include string literals in the app's JS code, as well as HTML served on the initial page load.
React TodoMVC Demo
Nope, most websites will break if you try to inspect them with FromJS.
FromJS uses Babel to add tracking to strings, but this results in a lot of flakiness.
Non-trivial apps also take a long time to load and use up large amounts of memory.