If a set line-height is smaller than font-size, then glyphs bleed out of the line box and then content of contiguous line boxes overlap.

This 1st sentence has a set font-size: 20px inside a set line box height of 10px.
This 2nd sentence has a set font-size: 20px inside a set line box height of 10px.
This 3rd sentence has a set font-size: 20px inside a set line box height of 10px.