The attributes allowing an embedded video (e.g. Vimoe video) to be displayed in full screen (e.g. webkitallowfullscreen, mozallowfullscreen, allowfullscreen) are removed when the embedded code is used in a Content block widget.

Here is a sample code which can be placed in the HTML mode of the Content block widget:

 

After the changes are saved or the user switches to Design mode, the allowfullscreen attributes are removed. This is the reason why when accessing the page with the embedded video on the frontend it cannot be displayed in full screen mode.

By default the Content block widget modifies all html5 tags and attributes in order to keep support for old browsers.

In order to change the default behavior the ConvertToXhtml filter should be disabled.

In order to disable the filter perform the below steps:

1. Go to Administration -> Settings -> Advanced -> Appearance
2. Find the following fields:
   Rad Editor's content filters
   RadEditor filters for Content block widget
3. Enter the following filters in the above fields:
   
   RemoveScripts, EncodeScripts, StripCssExpressions, StripDomEventAttributes, FixUlBoldItalic, IECleanAnchors, FixEnclosingP, ConvertTags, ConvertFontToSpan, OptimizeSpans, IndentHTMLContent, ConvertCharactersToEntities, PdfExportFilter, ConvertInlineStylesToAttributes
   
   The above filters include all the default filters except the ConvertToXhtml.
4. Save the changes

Refer to the below article for more details on how to disable Rad Editor filters: http://www.sitefinity.com/developer-network/knowledge-base/details/how-to-stop-htmleditor's-filters