Question

What is the essential difference between:

position: absolute;
top: 0;
height: 100%;

and

position: absolute;
top: 0;
bottom: 0;

Answer 1

The height of the child element is determined differently for each property:

bottom: 0 =>
child_height = parent_height – child_margin – child_border

height: 100%=>
child_height = parent_height

In other words height: 100% sets the inner height of the child to the same height of its parent, while bottom: 0 sets the outer height of the child to the same height of its parent.

Example from https://jsfiddle.net/2N4QJ/1/

.parent {
    width: 100px;
    height: 300px;
    position: relative;
    background: #ccc;
    display: block;
    float: left;
    padding: 10px;
    margin: 20px 
}

.parent > div {
    display: block;
    margin: 10px;
    background: red;
    position: absolute;
    color: white !important;
}

#c1 {
    top: 0;
    background-color: green;
    height: 100%;
}

#c2 {
    top: 0;
    background-color: blue;
    bottom: 0;
}

<div class="parent">
    <div id="c1">height: 100%, margin: 10px</div>
</div>

<div class="parent">
    <div id="c2">bottom: 0, margin: 10px</div>
</div>

More info about position/dimension:
http://msdn.microsoft.com/en-us/library/ms530302(VS.85).aspx (archive link)