preserve-3d rotationY doesn't appear to work

[drok]

Hi!

Thank you for the brilliant SVG technique - I tried applying it to an animated logo I'm making, but can't quite get a rotationY animation to work - the PR is my attempt.

When displayed on its own, the SVG plays as I would expect, but when embeeded, the backface seems to not be hidden, causing the backside text to appear overlayed on the frontface, as seen below:

Embedded into readme:

Standalone SVG:

https://raw.githubusercontent.com/drok/css-in-readme-like-wat/exp28/header.svg

However, embedding the SVG into README.md breaks the animation - any idea why?

I also note that the "distributed digital library" text flashes in the README.md at the beginning of the animation, but does not flash in the standalone image

Thanks for any insight
Cheers