WordPress supports YouTube links converting them into embedded video. You can even do this with comments. Embedding is very easy, you just add the link to a new line and you’re done.

Well… almost, because it might break your design. By default it renders an iframe with 900 height and 506 width, which is neither responsive nor — in my case — the right size:

<p>
<iframe width="900" height="506" 
        src="https://www.youtube.com/embed/5U96CmovF_w?feature=oembed" 
        frameborder="0" allowfullscreen></iframe>
</p>

WordPress allows you to specify the height and width as query string parameters, but that would still not make it responsive. I’ve came up with some CSS to fix this issue (at least for modern browsers). All my video-posts are tagged ‘video’, so I only tweak iframes inside video. This prevents messing up other stuff:

.video iframe
{
	min-width:100%;
	max-width:100%;
	max-height:400px;
	margin-top:-1em;
}

Because WordPress generates a paragraph tag, I needed to add a negative top margin to get rid of the break. Check the result: