Diff view is not there on CSS Challenges, which is perfectly mitigated by this userscript! It's a r###lt of my comparison of cssbattle.dev and css-challenges.com.

Just hover on your r###lt (left frame).

Moving the mouse left and right will move the delimiter, just so you can observe both r###lt and preview parts.

Moving the mouse up will make the delimiter appear, moving down will make it disappear.

Embrace the pixel perfect challenge!