text-transform doesn't work on non-BMP characters (such as Deseret) because nsCaseTransformTextRunFactory::RebuildTextRun and the code it uses operates on 16-bit code units.
This makes us fail:
Created attachment 602828 [details] [diff] [review]
patch, handle surrogates in text-run case transformation
This depends on bug 210501 to provide the support for case-mapping of non-BMP character codes; once that infrastructure is done, we can fix text-transform to handle Deseret properly. :) (And any future casing alphabet that may get added to Plane 1...)