Best practices for HarmonyOS application development with ArkTS and ArkUI, covering components, state management, resources, layout, lifecycle, and accessibility. Use when writing .ets component files, using ArkUI decorators like @Component/@State/@Prop/@Link, building layouts with Column/Row/Stack/List, managing HarmonyOS app resources with $r(), or handling component lifecycle and accessibility on HarmonyOS. It provides structured guidance on component architecture, state propagation, resource externalization, and responsive layout styling. Reach for it when building declarative user interfaces for HarmonyOS applications and needing to follow established framework patterns and conventions.