Just like code smells in source code, UI design has visual design smells. We study 93 don’t-do-that guidelines in Material Design, a complex design system created by Google. We find that these don’t-guidelines go far beyond UI aesthetics, and involve seven general design dimensions (layout, typography, iconography, navigation, communication, color and shape) and four component design aspects (anatomy, placement, behavior and usage). Violating these guidelines results in visual design smells in UIs (or UI design smells). In a study of 60,756 UIs of 9,286 Android apps, we find that 7,497 UIs of 2,587 apps have at least one violation of some Material Design guidelines. This reveals the lack of developer training and tool support to avoid UI design smells. To fill this gap, we design an automated UI design smell detector (UIS-Hunter) which extracts and validates multi-modal UI information (component metadata, typography, iconography, color and edge) for detecting the violation of diverse don’t-guidelines in Material Design. The detection accuracy of UIS-Hunter is high (precision=0.81, recall=0.90) on the 60,756 UIs of 9,286 apps. We build a guideline gallery with real-world UI design smells that UIS-Hunter detects for developers to learn best Material Design practices. Our user studies show that UISHunter is more effective than manual detection of UI design smells, and the UI design smells that detected by UIS-Hunter have severely negative impacts on app users.