Find which Safari window is playing video? Safari 9.0.1 launches and restores previously open windows. The app has the rude default behavior of automatically restarting any open videos. 

With many open windows and tabs, how can I find the window playing the video?

In Chrome, I can use Window > Task Manager to view a list of processes. The busy one has the video. Double-clicking brings that window to the front. Anything similar in Safari?
In Safari 9 (and probably later), a blue speaker icon appears in the URL bar of all windows whenever any window is playing audio. You can click on this to mute the current tab (if it's playing audio) or all other tabs (if the current one is playing audio). 

This icon also has a hidden feature: If you click and hold on it, you will see a list of all the tabs that are currently playing sound. Click on one of those, and the corresponding window will be brought to the front. Then you can stop that particular video.