Margin collapsing
Also written: collapsing margins
When the bottom margin of one block and the top margin of the next overlap and the larger one wins, so 20 px and 30 px give a 30 px gap, not 50.
Also written: collapsing margins
When the bottom margin of one block and the top margin of the next overlap and the larger one wins, so 20 px and 30 px give a 30 px gap, not 50.