Chrome's support for the keys will take place at the browser level rather than the tab level, which means multimedia buttons will work even when Chrome is operating in the background or minimized. If you're watching a YouTube video and you pull up another app while the video is playing, hitting the pause button will still stop it. You can see the feature in action on a demo page set up by the Chrome team or test it out in the Chrome 73 Beta and Chrome Canary builds that are available now.
Initial support for multimedia keys within Chrome 73 will include commands for "play," "pause," "previous track," "next track," "seek backward," and "seek forward." It will also ship with a Media Session API that will let developers customize how their sites and apps interact with multimedia keys. Chrome will be the first browser to offer such support.