The second parameter (second item in each of the parentheses) is the version of the button to display after the event (the mouse rolling over the first image) has taken place, and so, onMouseover ...

Start playing a video on a page, and then hover your mouse over it. A small blue “Picture in picture” button should appear over the video; click the button and the video will swap to PiP mode.

A couple of days ago, Google removed the "View Image" button when viewing image results in Google Images. Of course, this makes it harder to view full-sized images but there's already a Chrome ...

As spotted by Engadget, the pop-up player is currently limited to an unknown number of users via a picture-in-picture button on the bottom right-hand side of the web app.