Proportional poster images (IE9+ Chrome, Safari, Firefox)

Added the poster image as the CSS background image to the .mejs-poster div, and hid the original <img> from all browsers except IE8 (where it's still 100% wide and high) and below with the following CSS

    :root .mejs-poster img { display: none ; }

Added the following CSS to constrain the background image to the div:

    .mejs-poster {
        background-position: 50% 50% ;
        background-repeat: no-repeat ;
        background-size: contain ;
    }

That should keep the poster image constrained and centred vertically and horizontally, so that it covers up the video element nicely.