if u want to change the video view in website u can use <iframe> tag instead of <video> tag
<iframe width="420" height="315" src="youtube.com/embed/XGSy3_Czz8k </iframe>
The video element allows a browser to play a video natively, as opposed to a plugin like flash, an iframe allows you to load the source of another url into your page.
In this video here from the course, the iframe is not playing the video. It's only loading a url which contains code that is playing the video.
The reason an iframe was used is because that video was being hosted on vimeo and the iframe code is required in order to embed the vimeo video onto your own web page. So the iframe isn't responsible for playing the video but rather it's responsible for loading the source of a vimeo url onto your own page. That source that is inside the iframe will either contain an object/embed code for flash or it might contain the html5 video element depending on what browser you're using.
You'll be able to see this if you use your browser dev tools to inspect the html. You can drill down into the iframe and eventually find the actual code that's playing the video.