'There is no attribute "allowfullscreen"' iFrame embedded YouTube videos

Problem You've embedded a video from YouTube into your HTML using their embed code e.g. <iframe width="560" height="315" src="https://www.youtube.com/embed/dNVpL2-8-BU" frameborder="0" allowfullscreen></iframe> You then validate your HTML and get the error: there is no attribute "allowfullscreen" Cause As stated by the w3c validation tool , one of the likely causes is that you're using a document type that does not support the "allowfullscreen" attribute for this element. The w3c validation tool tells you which document type you're using: In this case, we're using XHTML 1.0 Transitional which does not support allowfullscreen. Solution 1 HTML5 does support the allowfullscreen attribute on the iFrame element so changing the doctype to HTML5 will fix this issue.  You can try this from the w3c validation tool by changing the drop down just to the right of doctype from (dete...