Base UI Handbook: Foundations

Use this skill when a question is cross-component and not about one concrete component’s API.

Scope

• Styling model (className, style, render, state callbacks)
• CSS hooks (data-* state attrs, CSS vars, data attributes from popup placement)
• Animation and transition attributes
• Composition patterns with nested render callbacks
• mergeProps, mergePropsN, and useRender for wrapper authors
• Customization and behavior tuning (open, onOpenChange, reason-based handlers, cancellation helpers)

Routing cues

• "style hooks", "Tailwind", "CSS module", "composition", "render prop", "mergeProps", "useRender", "animation", "transition", "custom behavior", "event reason" → this skill

Canonical references