Re: Version 0.3+63eef9f

// @grant       	none
// ==/UserScript==

function GM_addStyle("				\
    video.webm { 			\
        max-width: 100%;	\
        height: auto;		\
    }						\
");

Assuming this script is still maintained this section of code should be at least @grant GM_addStyle and not @grant none.

Thanks for the look.

Also you should probably remove the word function in front of GM_addStyle for it to work.