Srcset Validator

This tool analyzes a given srcset and optional sizes attribute to verify that candidate images are declared correctly and that the browser can select appropriate assets across viewport widths and DPRs. It is intended for web developers, performance engineers, and SEO specialists who implement responsive images and rely on predictable asset delivery. The validator operates as a static analysis step, not a live fetch, and emphasizes correctness, coverage, and compatibility with the HTML5 spec.
Conceptually, the tool tokenizes the srcset into candidates and reads descriptors (w for width or x for density). It computes feasible target widths by combining the viewport width with DPR and checks for missing descriptors, malformed URLs, duplicates, or candidates that would never be chosen in typical viewing scenarios. The process yields a structured report that highlights gaps and recommends a minimal, robust subset of sources to deploy.
Outputs include a validity flag, a list of issues with rationale, and suggested replacements or additions. The tool can be used in CI to enforce correctness before merging, or during audits to inform asset strategy. By focusing on correctness and coverage rather than performance metrics, it helps teams avoid under-delivered images or wasted bandwidth while improving accessibility and search performance.