Relative parent, absolute positioning vertically by percentage?

I'm trying to create a vertically positioned DIV by percentage. I have the parent container to set to relative and the content div set to absolute. This works fine when I position the content div with pixels, but when I try percentages the percentages are disregarded:

.container {
position: relative;
}


.content {
position: absolute;
left: 10%;
top: 50%;
}


<div class="container"><div class="content"> This is the content div. It should be 10% from the left of the container div.
</div></div>

The content div appears at the top of the page, disregarding the 50% vertical placement. What am I missing? Thanks in advance!