Allow embedded YouTube video to go fullscreen

Have you ever seen this little annoyance?

YouTube's help section tells you very little about how to solve it.

Thankfully, it's pretty simple.

Assuming you've been given some HTML to embed into your website, such as:

<iframe src="https://www.youtube.com/embed/?list=UULFZXXi0dM4r7-sKFNo1kGlwg" width="480" height="400"></iframe>

Then you only need to add one attribute, allowfullscreen:

<iframe allowfullscreen src="https://www.youtube.com/embed/?list=UULFZXXi0dM4r7-sKFNo1kGlwg" width="480" height="400"></iframe>

And that's it! It works!