How to Verify AI-Written UI Changes Across Viewports

A study of ten first-pass AI-generated UI components found that all passed checks at three desktop widths but all failed at 360px and 390px mobile widths, indicating that viewport size, rather than browser choice, was the key factor exposing responsive design defects. Across 120 Chrome and Edge renders, mobile failures included elements extending beyond the viewport, horizontal overflow, and undersized interactive targets that violated WCAG’s 44-by-44-pixel pointer-target guidance, while some defects produced no detectable page overflow. The most common causes were non-wrapping flex rows with fixed-width children, tables containing unwrappable content, and controls sized for desktop mouse use. The findings recommend adding a 360px-to-390px render to pull-request checks, optionally adding a 768px tablet check for multi-column layouts, and using element geometry assertions alongside visual baselines to detect and prevent regressions. The study notes its limits, including its small sample, single AI agent, standalone components, and use of only two Chromium-based browsers, but argues that mobile rendering should be a routine verification step for AI-written interfaces.

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