Resources
Artists & Developers
yui540
One of the greatest CSS stage transition author I know is yui540, he/she designed so many other stunning looking ACG websites, and the most famous one was 臆病な魔女 (source code can be found under yui540).
To achieve the similar transition effect like the above, you may reference to this repository yui540/css-animations: 俺流CSSアニメーション, it has a live demo you can play around too.
Nihe Works
He/she posts videos of his/her works on YouTube too, you can check them out.