height 100% doesn't work, two divs inside div

Answer 1

because your parent div does not have property of height with height :100% you should have height:specific height on parent element,but you have min-height ,if the height is dynamic you should use jQuery to do this

Answer 2

it doesnt get height because the parent div has no height. use below code

<div style="height:150px; border:solid 1px black; float:left;">
    <div style="border:solid 1px black;height:100%;  float:left;">
    First to set height
    </div>

    <div style="border:solid 1px red; height:100%; float:left;">
    Why don't get the 100%
    </div>
</div>