On my WebKitGtk build with libsoup backend, I'm unable to open local (i.e. on my hard disk or other storage media) HTML files in Epiphany, load gtk-docs in (patched) devhelp and load user docs in (patched) Yelp.
A bug against libsoup was opened asking to add support to data: and file: protocols, but it was also closed as WONTFIX, suggesting to use gio/gvfs for any non http: stuff.
PS please note that gio/gvfs should also be useful to manage other protocols in browser links, such as mailto: and any other non-Web protocol used to open or start other applications.
PPS yes, maybe "Add GIO/GVFS support to WebKitGtk" was a better summary...
Sorry for this duplicate, the search page don't show unconfirmed bugs by default, so i didn't see previous report...
*** This bug has been marked as a duplicate of 18490 ***