Just like code smells in source code, UI design has visual design smells. We study 93 don t-do-that guidelines in the Material Design, a complex design system created by Google. We find that these don t-guidelines go far beyond U I 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 U I 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 U I design smells. To fill this gap, we design an automated UI design smell detector (UIS-Hunter) that extracts and validates multi-modal U I 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 U I design smells that UIS-Hunter detects for developers to learn the best Material Design practices. Our user studies show that UIS-Hunter is more effective than manual detection of UI design smells, and the U I design smells that are detected by UIS-Hunter have severely negative impacts on app users.