negative margin
Negative top and left margins move the element up and left while negative right and bottom margins make following siblings move left and up.
The negative top value moves the yellow box upward, but the positive bottom value keeps the following box in place.
The box styled as suggested below would display the same, but would also create a different stacking and positioning context (because of position:relative).
Style the yellow box with:
position: relative;
top: -100px;