Created attachment 263480 [details] reduction showing the described bug If a text is rendered with an OpenType font supporting the "liga" feature (e.g. Source Sans Pro) and this text also contains soft hyphens, strange characters are displayed around the soft hyphens following a ligature on the same rendered line. If ligatures are disabled by -webkit-font-variant-ligatures: no-common-ligatures; or -webkit-font-feature-settings: "liga" 0; text is displayed correctly.
Please try in a nightly build. I think I fixed this in http://trac.webkit.org/changeset/190891
I tested the above in WebKit-SVN-r191175 and now in WebKit-SVN-r191340: no changes. Although I'm not sure if this is really the nightly build since "About Safari" says "Version 9.0 (11601.1.56)" when I launch the nightly build. Is that normal or did I make a mistake?
Unfortunately, nightly builds don't currently work on OS X El Capitan. As far as I know, they should work if you temporarily disable System Integrity Protection.
It's very unfortunate that there's no note about nightlies not working on El Capitan on nightly.webkit.org I used the workaround on https://trac.webkit.org/wiki/WebKitNightlyElCapWorkaround and now I can confirm that this bug has been fixed indeed. Thanks for the fix and sorry for the double post. Kind regards, Mathias