You will need to install a user script manager extension to install this script.
Diff view is not there on CSS Challenges, which is perfectly mitigated by this userscript! It's a result of my comparison of cssbattle.dev and css-challenges.com.
Just hover on your result (left frame).
Moving the mouse left and right will move the delimiter, just so you can observe both result and preview parts.
Moving the mouse up will make the delimiter appear, moving down will make it disappear.
Embrace the pixel perfect challenge!