Quick note of intention
Remove <img> from .mejs-poster and set it as the background image instead
Set background image of .mejs-poster to be positioned in the centre and sized to contain
Build
Fixed background-repeat setting (no-repeat)
Put poster image back, but hide it with CSS
Rebuild
Hide poster <img> from IE9+ and modern browsers by using :root hack
Merge pull request #838 from CAGD/master

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