<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" src="//www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
</div>
You can customize aspect ratios with modifier classes:
<!--21:9 aspect ratio-->
<div class="embed-responsive embed-responsive-21by9">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<!--16:9 aspect ratio-->
<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<!--4:3 aspect ratio-->
<div class="embed-responsive embed-responsive-4by3">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<!--1:1 aspect ratio-->
<div class="embed-responsive embed-responsive-1by1">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<div class="float-left">float left on all viewport sizes</div><br>
<div class="float-right">float right on all viewport sizes</div><br>
<div class="float-none">Don't float on all viewport sizes</div><br>
<div class="float-sm-left">float left on viewports sized SM (small) or wider</div><br>
<div class="float-md-left">float left on viewports sized MD (medium) or wider</div><br>
<div class="float-lg-left">float left on viewports sized LG (large) or wider</div><br>
<div class="float-xl-left">float left on viewports sized XL (extra-large) or wider</div><br>
// Related simple non-responsive mixins
.element {
@include float-left;
}
.another-element {
@include float-right;
}