I love the Epiphany browser's look and aesthetic, but when using pages with a lot of multimedia content, it will eventually lock up, or crash. I already have gstreamer1.0-libav installed. I'm currently using Firefox, but would like to be able to use the default browser, but it currently has too many issues. Youtube plays fine, but other sites, like Musecore.org tutorials, and my school's website don't work properly.
If it's crashing, then you should file a bug report with a quality backtrace so it can be investigated. Crash reports are much appreciated as long as they contain a good backtrace. Nobody can assist with your crash otherwise.
As for the hangs... unfortunately that is a known issue. It will be half-fixed in the forthcoming WebKitGTK 2.24.4 release, but the other half of the fix will not be available until GStreamer 1.16.1 at the earliest (and maybe not even until GStreamer 1.18, it's still uncertain). Since elementary OS will almost certainly not update GStreamer past the version available from Ubuntu, than means it will unfortunately remain broken until the next major version of elementary OS.
The only good news here is the next OS version should include Epiphany as a flatpak instead of a deb package, so important fixes should land much more quickly.
Edit October 2019: The fix is available in GStreamer 1.16.1.