There may be easier ways of solving this, but I used a bit of javascript from CSS Tricks in the footer.php file, right after wp_footer();
<script>
// By Chris Coyier & tweaked by Mathias Bynens
jQuery(function() {
// Find all YouTube videos
var $allVideos = jQuery("iframe[src^='https://www.youtube.com']"),
// The element that is fluid width
$fluidEl = jQuery(".entry-content");
// Figure out and save aspect ratio for each video
$allVideos.each(function() {
jQuery(this)
.data('aspectRatio', this.height / this.width)
// and remove the hard coded width/height
.removeAttr('height')
.removeAttr('width');
});
// When the window is resized
// (You'll probably want to debounce this)
jQuery(window).resize(function() {
var newWidth = $fluidEl.width();
// Resize all videos according to their own aspect ratio
$allVideos.each(function() {
var $el = jQuery(this);
$el
.width(newWidth)
.height(newWidth * $el.data('aspectRatio'));
});
// Kick off one resize to fix all videos on page load
}).resize();
});
</script>