'GNOME Help' suggests Moonlight plug-in, which recommend non-free software.

The Moonlight player / plug-in itself is free software; (All the relevant source code is licensed under the LGPLv2, Microsoft Public License, or MIT license.) But it requires the user to download proprietary software. (The Microsoft Media Pack.)

Trisquel GNU/Linux package repository no more hosts the moonlight package for the aforesaid reason.

Unfortunately GNOME 3 documentation still recommends Moonlight plug-in instead of Silverlight from Microsoft. (See the attachment for details.)

Please Remove the entire section named "Install the Silverlight plug-in" (Main Help page >> Networking, web, email & chat >> Web Browsers >>) from GNOME 3 documentation.