Warning: attempting to read this page will lead to increased anxiety, decreased productivity, and a strong urge to yell at your computer.
Under-Optimizing for Beginners (Don't Bother)
1. Micro-optimization of pixel art: because 1000px images are clearly too big.
2. Recursive function calls for every single calculation: because who needs performance?
3. Using 10 different CSS frameworks for the same layout: because who needs simplicity?