You will need to install a user script manager extension to install this script.
Unfortunatley Apple removed support for Safari Internet plug-ins so this no longer works.
Safari no longer supports most plug-ins. To load webpages more quickly, save energy, and strengthen security, Safari is optimized for content that uses the HTML5 web standard, which doesn't require a plug-in. To enhance and customize your browsing experience, you can use Safari Extensions instead.
Lets Safari play embedded WebM videos on supported imageboards: 4chan, 8chan, Gelbooru and Danbooru.