html : iframe not showing content in html

You should change the youtube URL to remove the "s" so it looks like this:

<iframe width="420" height="345" src="//www.youtube.com/embed/C8kSrkz8Hz8" frameborder="1" allowfullscreen></iframe>

You should also be able to embed a youtube video from the page. Please see the screenshot attached:

When you try to put the whole YouTube page into an iframe, it sends a HTTP header called X-Frame-Options with the SAMEORIGIN value, which tells the browser, that the page can only be displayed in a frame on the same origin as the page itself.

You should use the provided embed code (you can find it below every YouTube video), which is also an iframe, but with a different URL. It will only show the player.

In this case, the embed code would be:

<iframe width="560" height="315" src="//www.youtube.com/embed/C8kSrkz8Hz8" frameborder="0" allowfullscreen></iframe> 

replace you Iframe with this one. this works for you.

<iframe width="640" height="390" src="//www.youtube.com/embed/C8kSrkz8Hz8" frameborder="0" allowfullscreen></iframe>

missing attributes are: frameborder="0", allowfullscreen

Change your src with //www.youtube.com/embed/C8kSrkz8Hz8

your code shoud look like this

<iframe frameborder="1" width="420" height="345" src="//www.youtube.com/embed/C8kSrkz8Hz8"></iframe> 

Find code under each video on youtoube at Share menu.