Created attachment 228270 [details] Test file Load iframe2.html in minibrowser and instead of a green rectangle, you will see a green rectangle and a red rectangle because the iframe content is clipped at the region's boundaries. If you load the same test in WK2, the test works fine.
Created attachment 228272 [details] Iframe content for test To be used with iframe2.html uploaded before
CSS Regions were removed in Bug 174978.