Never start designing immediately. Spend the first minutes gathering requirements.
Functional requirements: What features must exist? What can users do?
Non-functional requirements:
- Performance targets (load time, interaction latency)
- Accessibility needs (WCAG level)
- Internationalization (how many languages?)
- Offline support needed?
Constraints: Which browsers? Mobile or desktop? Network conditions?
Interview tip: Write requirements on the whiteboard. Reference them throughout to show you're designing to spec.