Created attachment 266236 [details]
simple web page showing the problem
On Windows, some web fonts(e.g. Droid Serif, Titillium Web) aren't rendered with 'text-rendering: optimizelegibility' CSS value. 
After some debugging, I found that ScriptShape(), which is in UniscribeController::shape(), returns E_FAIL with those problematic fonts. (I couldn't find any reasons for the failure)
It seems that when it fails, there is no fallback mechanism, so no texts are displayed on the screen.
It would be helpful if somebody look into this.
E_FAIL -> E_FAIL
The best course of action would be to create a reduced test case and submit a bug report to Microsoft for Uniscribe.
I agree. I'm finding a way how to file a bug for Windows.
Created attachment 266425 [details]
visual studio project showing the uniscribe bug
A simple Windows program that displays "AB" using Uniscribe.
You can change the font through the menu, "View>Font".
Make sure install "Droid Serif.ttf", "Titillium Web.ttf" so that they can be displayed on the font-pickup dialog.
I filed a bug to MS.