If you’re using Fedora and discovered that WebKitGTK+ is displaying blank pages, the cause is a bad mesa update, mesa-18.2.3-1.fc29. This in turn was caused by a GCC bug that resulted in miscompilation of mesa.
To avoid this bug, downgrade to mesa-18.2.2-1.fc29:
$ sudo dnf downgrade mesa*
You can also update to mesa-18.2.4-2.fc29, but this build has not yet reached updates-testing, let alone stable, so downgrading is easier for now. Another workaround is to run your application with accelerated compositing mode disabled, to avoid OpenGL usage:
$ WEBKIT_DISABLE_COMPOSITING_MODE=1 epiphany
On the bright side of things, from all the bug reports I’ve received over the past two days I’ve discovered that lots of people use Epiphany and notice when it’s broken. That’s nice!
Huge thanks to Dave Airlie for quickly preparing the fixed mesa update, and to Jakub Jelenik for handling the same for GCC.