automatic zoom on iphone safari

Is there a way for Mobile Safari to recognize a site meant for mobile phones and automatically zoom in? How does that work on other phones and browsers?

I'd like to avoid looking at user_agent and sending a different page for each mobile browser.

How do I fix the zoom on my iPhone Safari?

Adjust the magnification: Double-tap the screen with three fingers (without lifting your fingers after the second tap), then drag up or down. Or triple-tap with three fingers, then drag the Zoom Level slider.

Why does Safari suddenly zoom?

Sounds like you have system zoom enabled. You can disable it from System Settings > Accessibility > Zoom.

See the configuring the viewport part of the Apple web-app developer guide.