Apple iPhone 16: Screen Size & CSS Viewport

The iPhone 16 brought Camera Control — a new hardware button — to the standard iPhone, democratizing quick camera access for the first time. From a responsive testing angle, what matters most about the iPhone 16 is its CSS viewport: 390×844 pixels, the most important mobile viewport dimension in existence.

The 390px width is the de facto standard for iPhone-first mobile responsive design. It is the viewport that most designers target when they say "mobile view," the width referenced in countless design system tokens, and the primary viewport for Apple's own HIG guidelines for web content. Any mobile optimization work that does not explicitly test at 390×844 has a measurable gap in its coverage.

The A18 chip dramatically improved JavaScript execution speed on the base iPhone. Complex client-side rendering, heavy animations, and large single-page applications that previously caused frame drops on standard iPhones now run significantly smoother. This shifts the performance baseline upward — animations that were acceptable at 60fps on the A16 are now expected to maintain 60fps consistently on the A18, since dropped frames are more noticeable against a smooth baseline.