There are 2 issues, the second issue I actually found when trying to reproduce the first one.
1) When resizing a TextBlock with a custom font that was embedded via css it is cropped and/or rendered badly. (Size is dependent on idealWidth so resize the window to test)
2) Embedded font doesn't render in bjs even though it is clearly loaded. In my local project it is rendered properly but for some reason in the repro it uses the default font instead.
I tried to set a long timeout to make sure the font was loaded (although DOMContentLoaded should suffice), still no luck.
Once this second issue is solved you'll be able to test the first one.
Repro: