There is a conflict between this restriction

"Dynamically modifying a source element and its attribute when the element is already inserted in a video or audio element will have no effect. To change what is playing, just use the src attribute on the media element directly, possibly making use of the canPlayType() method to pick from amongst available resources. Generally, manipulating source elements manually after the document has been parsed is an unnecessarily complicated approach."

and the shadow DOM distributions spec

http://w3c.github.io/webcomponents/spec/shadow/#distributions

that results in the video tag not being able to be constructively used inside a web component

Example here: http://jsfiddle.net/n88owmop/4/