Periodicially, the city of Juneau, Alaska, sets standards for allowed 'On Sale' items in the tourist stores (what we locals call generically: T Shirt Shops). They decree that it is NOT allowed to blare 50% Off!, 80% Off! while not making it clear what the NOT on sale price was.

They levy a hefty fine per day on stores that persist in the practice.

Last summer I noticed that a number of stores switched to advertising: "80% off lowest price listed on item". That sounds fair to me.