Whitespace
Characters that appear as empty space: space, tab, newline, carriage return, vertical tab. In HTML/CSS, multiple whitespace collapses into one. In Python, indentation is significant.
CSS white-space: pre preserves whitespace. Whitespace-sensitive languages: Python, YAML, Haskell, Makefile (tab vs space). Whitespace-insensitive: C, JS, JSON. Python PEP 8 says 4 spaces, not tabs. Make requires tabs and refuses to work with spaces — classic trap. There's even an esoteric language called "Whitespace" where only spaces, tabs, and newlines carry semantic meaning.