I'm not sure what exactly you want, but if I get it right you want to have the high score for each level and a total score that is the sum of all level scores, right?
In this case, you'd have several Webstorage objects (scoreLvl1 ... scoreLvlN). Every time a player finishes a level you'd check if their score for the level is higher than the Webstorage value for that level. If it is, you'd set the Webstorage to the new value.
For the total score you'd just use the sum of every Webstorage object.