Rule 1: Never trust a pixel when it's 2px shy of the edge.
Rule 2: Always wear your best black on a black background.
Rule 3: Never click on a link with less than 4px of whitespace.
Rule 4: Use Comic Sans for all formal events, it's a classic.
For more rules, visit 01 - The Uncanny Valley of Font Sizes
For the opposite of this page, visit Anti Etiquette