CSS – why doesn’t percentage height work? [duplicate]

How come a percentage value for height doesn’t work but a percentage value for width does?

For example:

<div id="working"></div>
<div id="not-working"></div>

#working{
    width:80%;
    height:140px;
    background:orange;
}
#not-working{
    width:80%;
    height:30%;
    background:green;
}

The width of #working ends up being 80% of the viewport, but the height of #not-working ends up being 0.