Best method to center a responsive div horizontally and vertically in a a 100% div

Question

How do I horizontally and vertically center a dynamic div in a div with 100% viewport size?

html

<div class="outer-div">
    <div class="inner-div"></div>
</div>

css

html, body {
    height: 100%;
}

.outer-div {
    height: 100%;
    width: 100%;
    background-color: #f3f3f3;
}

.inner-div {
    width: 80%;
    height: 80%;
    background-color: #ccc;
}

Answer 1

Demo

css

html, body {
    height: 100%;
    margin:0;
    padding:0
}

.outer-div {
    height: 100%;
    width: 100%;
    background-color: #f3f3f3;
}

.inner-div {
    width: 80%;
    height: 80%;
    background-color: #ccc;
    margin:auto;
    position: absolute;
    top:0;
    bottom:0;
    left:0;
    right:0;
}

Answer 2

The important part is

position: absolute;
top:0;
bottom:0;
left:0;
right:0;

that will apply to the first parent that has position: relative or to body if nothing else has it.