WordPress iOS App – Video Width 100%?

It seems like a no-brainer to allow for the 100% tag on HTML5 uploaded from the WordPress iOS app. Instead, the video is set to a specific height and width and then screws up the layout (non-responsive). I’ve searched around the web and everyone talks about how to make YouTube or Vimeo embeds fluid (responsive), but nobody seems to be talking about directly uploaded video. I’m dealing with a personal blog that has few users looking at it (family) so the convenience of uploading video directly into WordPress outweighs the pros of hosting video on YouTube or Vimeo. So – my question – is there a site-wide way to make all of the HTML5 video set to 100% width? I can’t find a plugin that will do this. Maybe something in functions.php? (I’m a novice). Thanks for any help and advice.