Submitting Player card


Hi, I am trying to submit player card ( having URL which can fit in any device. ) . I got the message Build your HTML page used in the iframe (referenced via twitter:player:stream) to be responsive, ensuring the video content fills the full width of any display area provided, across all clients.

I am not sure why I got the message, because the URL I given is fitting to any device. I used the following code

CSS : 
    video {

<video width="100%" controls>
    <source src="XXXX" type="video/mp4">
    Your browser does not support video

Is there anything wrong in my code ??