Multimedia: Fix the rewind in Webkit browsers
Multimedia: Fixed the progressbar in polyfill mode
Normalised the shortened form for extra large to 'xl'.
Merge pull request #4104 from thomasgohard/normalise-extra-large-naming

Normalised the shortened form for extra large to 'xl'
Merge pull request #4102 from LaurentGoderre/fixes-multimedia

Multimedia fixes