Web Vitals and Performance Lab

Capability fixtures producing known, bounded Web Vitals and performance-budget values so crawler tests can validate LCP, CLS, TBT/TTI, FCP, and TTFB capture plus per-category transferred-size buckets.

Capability fixtures, not hint fixtures: nothing in this group deliberately triggers a hint, and every registry entry carries an empty hint list by design. The expected crawler signal is a metric moving into a defined band relative to the calibration baseline from the same crawl, never an exact number.

Families:

• Calibration baseline: a minimal page with inline critical CSS only and no external resources; every other fixture in the group is asserted relative to this page’s values.
• Layout shift: a 192px banner injected as the first body child at a fixed 2,000ms, with 120vh of content below so the expected CLS is computed from geometry (192 / viewportHeight). The capture window must stay open at least 3,000ms.
• Largest Contentful Paint: a hero image injected into a pre-reserved container at a fixed 4,000ms, so LCP is lower-bounded by construction and CLS stays in the baseline band. Capture windows shorter than 5,500ms miss the LCP candidate entirely — that requirement is itself under test.
• Main-thread blocking: exactly three 300ms wall-clock busy loops separated by 100ms yields, run once only, bounding added main-thread work at 900ms per load regardless of CPU speed (TBT band 600–900ms).
• Time to first byte: a delay endpoint that sleeps exactly 2,500ms before the first byte (poor band), concurrency-capped at 8 with a 429 + Retry-After overflow; risky-manifest, direct entry only.
• Performance budgets: one deliberately heavy resource per transferred-size bucket — document, script, stylesheet, image, media, font, and third-party — served with identity encoding so transferred size tracks committed size. Timing metrics on budget pages degrade incidentally and are explicitly not asserted.

All vitals are lab measurements: absolute values depend on hardware and capture configuration, so fixtures pin the page-side contribution (a delay, a busy-loop duration, an element geometry, a byte count) and assert bands or orderings relative to the baseline.

Fixtures in this category

• Web vitals calibration baseline — vitals-baseline-fast (safe)
• Layout shift — late-injected banner — vitals-cls-late-banner (moderate)
• LCP — late-injected hero image — vitals-lcp-late-hero (moderate)
• TBT — three bounded long tasks — vitals-tbt-long-tasks (moderate)
• Performance budget — document bucket — vitals-budget-document (moderate)
• Performance budget — script bucket — vitals-budget-script (moderate)
• Performance budget — stylesheet bucket — vitals-budget-stylesheet (moderate)
• Performance budget — image bucket — vitals-budget-image (moderate)
• Performance budget — media bucket — vitals-budget-media (moderate)
• Performance budget — font bucket — vitals-budget-font (moderate)
• Performance budget — third-party bucket — vitals-budget-third-party (moderate)

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