Working String Manipulation (no *script used) (substring, substr, indexOf, contains, replace)

I use a very similar idea for the level editor here: New puzzle: Mind the gap - with level editor and the race replay here: RacetraCS: Car Racing Game - #7 by choicehacker

… but I was curious to learn how the persisted saves are done. I believe CS must use some Javascript (cookies?) manipulation - I should look into the CS source.