How to structure a non-trivial interactive application (100+ lines of JS, multiple UI states, user interactions) in a single HTML file without it becoming an unmaintainable mess. Single-file apps are ideal for personal tools, demos, prototypes, and utilities because they have zero build step, zero dependencies, work offline, and can be shared by email or dropped into any web server. But without structure, they quickly become spaghetti: DOM reads mixed with business logic, state scattered across DOM attributes and global variables, event handlers that directly manipulate other parts of the UI, and no clear flow of data.