---
term: viewport
category: surface-system
status: canonical
authority: runtime-specific
---

Viewport

The visible presentation region for a stack. A viewport may be a web DOM region, Android overlay region, or XR spatial region.

Allowed:
- viewport
- stack viewport

Forbidden:
- page
- screen

Notes:
- Viewports are runtime-specific.
- Stack semantics must remain identical across viewport implementations.