iframe, short for "inline frame", is the most common way that the allowfullscreen attribute is used. Thanks mostly to video sites like YouTube and Vimeo, videos embedded by third-party websites using an iframe can use the allowfullscreen attribute to enable a "fullscreen" button within the iframe.
Here is sample code for a YouTube video embedded using an iframe along with the allowfullscreen attribute:
<iframe width="560" height="315" src="http://www.youtube.com/embed/jofNR_WkoCE" allowfullscreen></iframe>
When displayed in a browser, the iframe contains a fullscreen button:
Here is another code example with a video embedded from Vimeo:
<iframe src="http://player.vimeo.com/video/38514156" width="560" height="315" frameborder="0" allowfullscreen></iframe>
And, as before, when the iframe is displayed in a web browser, it looks like this: