DocBook 5 embedded HTML iframe for YouTube with 'allowfullscreen'?

Dear all,

the standard HTML embed code for YouTube videos includes an attribute
setting 'allowfullscreen' , for example:

<iframe width="560" height="315"
src="https://www.youtube.com/embed/KySxOW7vVUc" frameborder="0"
allowfullscreen></iframe>

I included this in a DocBook 5 XML file using an external file and the
?dbhtml-include processor instruction. [see: Stayton book
http://www.sagehill.net/docbookxsl/InsertExtHtml.html]

The processor (Saxon 9) will not accept the bare 'allowfullscreen' as
it expects attribute settings to be in the form 'attribute = value'.

So I just modified the embed HTML to ' allowfullscreen = "true" ' and
it now processes and works correctly.

(Likewise, SlideShare embed HTML includes three attributes '
allowfullscreen webkitallowfullscreen mozallowfullscreen'.)

Does anyone know the definitive answer about this?

Thx Phillip