The vim-fugitive plugin comes with
the :Gbrowse command.
Open the current file, blob, tree, commit, or tag in your browser at the upstream hosting provider. If a range is given, it is appropriately appended to the URL as an anchor.
I would try this from repositories hosted on GitHub and get an error. Reading a
bit further into the :Gbrowse documentation I can see why.
Upstream providers can be added by installing an appropriate Vim plugin. For example, GitHub can be supported by installing rhubarb.vim, available at https://github.com/tpope/vim-rhubarb.
I was missing the vim-rhubarb plugin.
After installing that, I can open the current file in GitHub by running
:Gbrowse.
If I make a visual selection of any number of lines, then I can go into command
mode again typing out Gbrowse with the automatically preceding visual
selection tags. This will open the file on GitHub with those lines selected.
:'<,'>Gbrowse