YouTube is testing a new design that turns the bar of controls along the bottom edge of its video player transparent. The change is rolling out slowly, but if you are eager to get it now, you can do so by editing a cookie in your browser. Here's how to do it in Chrome and Firefox.
In Chrome, you will need to install a cookie editor, such as the EditThisCookie extension.
Next, head to YouTube and click the cookie button that EditThisCookie placed next to Chrome's URL bar.
Scroll down and find the line for VISITOR_INFO1_LIVE and click on it to expand it. In the box for value, replace what's there with Q06SngRDTGA and then click the big green checkmark at the bottom to save your changes.
Refresh YouTube and you'll see the new design. The bar with playback controls is transparent, and it disappears when you move your cursor off of the video player. It quickly reappears when you move your cursor back to the video player. It also remains visible when a video is paused.
In Firefox, you don't need to install an extension, you only need to open a page with YouTube and copy and paste a bit of code. On Windows, hit Shift-F2 to call up the Developer Console. On a Mac, hit Function-Shift-F2 to call up the Developer Toolbar or go to Tools > Web Developer > Developer Toolbar.
In the toolbar at the bottom of Firefox, copy and past this code: cookie set VISITOR_INFO1_LIVE Q06SngRDTGA and hit Enter.
To reverse course and return to YouTube's trusted opaque control bar, follow the instructions as outlined on Ghacks.