# How REP Works — Gateway Architecture and HTML Injection
> How the REP gateway injects environment variables into HTML at runtime. Proxy and embedded modes, startup sequence, script tag injection, and SDK initialization.
Source: https://rep-protocol.dev/concepts/how-it-works/
## Architecture
REP introduces a lightweight gateway process between the browser and your static file server. The gateway reads `REP_*` environment variables at startup, classifies them, and injects a `
```
**Injection rules:**
1. Insert before `` (preferred)
2. If no ``, insert after ``
3. If neither exists, prepend to the response body
> **Caution**
>
> `type="application/json"` is critical — it prevents the browser from executing the tag. This is inert data, not executable JavaScript. Never change this to `text/javascript`.
## SDK initialization
The SDK reads the injected payload **synchronously** on import:
1. Locate `