I am checking this out now.
On my desktop (Linux, KDE with libnotify), the notifications work correctly with Chromium (opensource upstream for the Chrome browser) and with Firefox. Both have the album art as well.
Interestingly, the Firefox notifications use the native libnotify notifications so they fit with the rest of the desktop. And they auto close after a few seconds. Chromium however uses its own format which has its own theme. It auto closes as well, but it is a longer time out. Not a deal breaker - it is just interesting!
It does not work with a QT5 Webengine widget (based on Chromium code), but this is normal since notification permission is denied by default with qtwebengine.
Go ahead a send a pull request. I will add this functionality in.
Once I pull it in, I will tweak a few things. I will likely change the “Theme” select button to a “Settings” button to open a settings overlay. And will probably tweak the wording of the Notify button a bit.
But thanks! This is good stuff!