This was a common technique in game development a years ago, back when gospel was no larger than 10MB for downloadable casual games. You can get great compress

This was a common technique in game development a years ago, back when gospel was no larger than 10MB for downloadable casual games. You can get great compression with GIF/PNG for alpha channel and JPG or JPEG2000 for color data. The downside is you usually get multiple compression artifacts on screen (assuming texture compression like DXT), which doesn't apply for web use.