CSS opacity - background colour

Question

I'm trying to get a black div's opacity to be .5 but the content of the div (h3 tag) to be opacity 1. So the white text is still white, with its opacity not changed/untouched.

<div style="background-color:red;">
<div style="width:470px;color:white;margin-top:170px;">
<div style="background-color:black;opacity:0.5;">
<h3 style="color:white;opacity:1;">Heading </h3><p>tagline here</p>
</div>
</div>
</div>