I've been searching for a while to figure out a way to get Safari to keep loading (buffering) paused videos when the tab containing the video isn't the one I'm currently focused on.
If I go to another tab in Safari to read something while the video loads, the video stops loading. But also if I leave the tab in the front and go to another application to do something while the video is loading the video stops loading.
This definitely happens every time when I'm dealing with Vimeo. I think it happens other times, but I can't think of any places to try out. I don't use flash player, so this might only be happening with html5 video players.
I found a page that promised to help with loading background tabs, but it relates to Lion (I'm on Yosemite) and the person seems to be saying that tabs don't load at all in the background. I'm not having that problem (and unsurprisingly the solution didn't work).