Single Responsibility for UI Molecules
The single responsibility principle from computer science applies directly to interface design: each component should do one thing and do it well.
A search form molecule combines a label, input, and button. That’s it. When combined, these atoms have purpose, the label defines the input, the button submits the form. The result is portable and reusable anywhere search is needed.
Burdening a single pattern with too much complexity makes it unwieldy. Simple molecules are easier to test, easier to reuse, and promote consistency throughout an interface.
The discipline isn’t restraint for its own sake. It’s that tightly-scoped components compose more reliably than sprawling ones.