I'm using Geary on Fedora 27 which depends on WebkitGTK4, updated recently from 2.18.0 to 2.18.1 based on this update: https://bodhi.fedoraproject.org/updates/FEDORA-2017-5659051dec
I immediately noticed that Arabic text rendering is incorrect as the letters disconnected from each other (kerning) but that happens in some fonts (TTF propably) not all of them. This update makes Arabic very hard to read, and I have to copy the text to Gedit to be able to read it. I downgraded to the previous version of WebkitGTK4 and everything went back to normal.
I opened a bug downstream in RHBZ, if you're interested in more details and screenshots: https://bugzilla.redhat.com/show_bug.cgi?id=1506590
*** This bug has been marked as a duplicate of bug 178625 ***