This property controls the size of the top margin of an element's rendering box. Negative values are allowed.
Margins are transparent and the background value of the parent element shines through.
If the adjacent margins are all negative, the larger of the negative values is used. Refers to a percentage of the width of the current element's containing block. At least one case of negative margins I tried also skews image dimensions from their intrinsic values. Applying this property to table cells is very problematic, especially negative margins. Some margin values cause extra cell-spacing while others cause cell and content overlap.