This solution is from Dave Rupert / Chris Coyier (I think). It requires a wrapper div but works pretty well.
HTML
<div class="iframe-rwd">
<iframe width="425" height="350" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="https://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=Seattle,+WA&aq=0&oq=seattle&sll=37.822293,-85.76824&sspn=6.628688,16.907959&t=h&ie=UTF8&hq=&hnear=Seattle,+King,+Washington&z=11&ll=47.60621,-122.332071&output=embed"></iframe><br /><small><a href="https://maps.google.com/maps?f=q&source=embed&hl=en&geocode=&q=Seattle,+WA&aq=0&oq=seattle&sll=37.822293,-85.76824&sspn=6.628688,16.907959&t=h&ie=UTF8&hq=&hnear=Seattle,+King,+Washington&z=11&ll=47.60621,-122.332071" style="color:#0000FF;text-align:left">View Larger Map</a></small>
</div>
CSS
.iframe-rwd {
position: relative;
padding-bottom: 65.25%;
padding-top: 30px;
height: 0;
overflow: hidden;
}
.iframe-rwd iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
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