NG: parent collapses to zero height without clearing floats

float: left (parent height is 0)
float: left

← This text overlaps with the floated elements

OK: clearfix restores the parent height

float: left (parent height is correct)
float: left

← This text appears correctly below the floated elements

NG: columns totaling 100% with borders cause overflow

50% + border
50% + border (overflows)

OK: box-sizing: border-box solves it

50% + border (fits)
50% + border (fits)