Features

User-visible (and developer-visible) capabilities that span multiple subsystems. These pages focus on what each feature does and where its code lives across the framework.

Page	Source paths	One-line summary
Content collections	packages/astro/src/content/	Typed access to markdown/MDX/data with the Content Layer API.
Server islands	packages/astro/src/core/server-islands/	Lazily server-rendered components inside an otherwise static page.
View transitions	packages/astro/src/transitions/	Built-in client-side router with the View Transitions API.
Internationalization	packages/astro/src/i18n/	Locale routing, fallbacks, middleware.
Image optimization	packages/astro/src/assets/	astro:assets, Sharp service, SVG optimizer.
Sessions	packages/astro/src/core/session/	Pluggable per-user state (memory, redis, …).
Actions	packages/astro/src/actions/	Type-safe server-side actions.
Middleware	packages/astro/src/core/middleware/	Per-request onRequest chain.
Dev toolbar	packages/astro/src/runtime/client/dev-toolbar/, packages/astro/src/toolbar/	In-browser dev tooling.
Fonts	packages/astro/src/assets/fonts/	Font subsetting and provider integration.
Content security policy	packages/astro/src/core/csp/	CSP headers and nonces.
Environment variables	packages/astro/src/env/	Typed astro:env virtual module.
Prefetch	packages/astro/src/prefetch/	Built-in prefetching of links.

Most of these features expose themselves through an astro:* virtual module. The set of virtual modules is enumerated in systems / Vite plugins.