On the REWE online shop you can maintain lists of favourites. You can also see whether a list contains any articles that are currently on sale. Unfortunately you have to manually go through that list to find those items.
With this script you get a switch that toggles the visibility of items that are not on sale, so you can focus on the ones on sale.
A very German acript, don't you think? ;-)

User Interface: you need to be logged in and in one of your favourites lists. Then you will see a toggle button to the right of the search box. Click it :-)