Adding a DBus interface would allow a lot of different things on Linux DEs.
MPRIS is a standardized interface to control media playback.
Having MPRIS capabilities in a browser would allow for global media playback control, which is already possible with nearly every Linux mediaplayer. Making this possible for media playback in browsers follows the trend of more and more applications/platforms becoming web-based.
- Pause/continue playback of a video without having to open the browser window and locate the corresponding tab
- Playlist control for e.g. YouTube or Spotify playlists
The playback control is done through e.g.:
- the task list's context menu
- a global MPRIS widget
- FN keys on laptops
- a smartphone paired with the DE running KDE Connect
Linux DEs, such as Gnome or KDE Plasma provide a central credentials management (Gnome Keyring, KWallet) which is exposed through the Secret Service interface. This would allow for a unified credential management without creating a secondary data silo within the browser itself.
Show the progress of ongoing up-/downloads in the DEs task list
Allow for integrating with DEs which have a global menu configured
Signaling the power management service through inhibition mechanisms to disable screensavers/screenlocking/suspend during media playback.
Creating an API to allow Browser Addons to make use of DBus would provide a lot of possibilities for far more usecases
Accessing the session's DBus services from the browser would probably require some kind of permissions handling (similar to the handling of webcams, microphones etc) to keep the user in control of what the browser is allowed to do.
I feel like this is the kind of capability that an embedder of Servo would support, not Servo itself.
It would be an interesting project to do some of this in Firefox, however. I've wanted MPRIS support in Firefox for a while.