Strapless should be forbidden.
It doesn't look good on anyone.
It looked quite OK on HM in the 50ties because she clearly wore a corset underneath and her boobs were thus fully supported.
But it does not look good when you have almost no bust (see Alex of Lux - she looks as if she is pressed into a tube).
I does not look when you have a small bust (because danger is that the style only enhances that you lack in that department, or your boobs are pressed even flatter).
It does not look good if you have bigger boobs (because they then tend to hang, or, if in a bandeau bra, you tend to show a "monoboob", or the whole thing looks sloppy).
It often doesn't even look good on women whose figure leaves nothing to wish, proportionwise, like Diana or Leti.
And it does not look good if you have broad shoulders - see Charlene.
A shawl or bolero certainly helps to make the style work better, as does a heart shaped cleavage, sometimes.
But from all pics above, I would maximum approve like 10 as really good. The rest would better have left with the designer.
The worst is maybe Fergie with the strapless + balloon skirt combo. Holy shit.