Conciseness

The body is mostly efficient with tight section structure, but there is real redundancy: the "Explain" section restates the intro's black-rectangle and perceived-performance points, and the Fix section repeats the Quick Reference's aspect-ratio and 1280×720 guidance verbatim in substance. This matches 'mostly efficient but includes some unnecessary explanation or could be tightened'; the duplication keeps it below a 4.

Actionability

Concrete, executable guidance dominates: a one-line code snippet (`<video poster='thumbnail.jpg' controls>`), specific numeric recommendations (1280×720 minimum, 640×360 for embeds, 16:9), and four explicit check criteria including URL resolution. Not a 5 because the body contains only a fragment rather than a complete copy-paste example (full examples are deferred to references/rule.md), leaving minor gaps.

Workflow Clarity

The Check → Fix → Explain → Code Review structure is a clear sequence, and the Check section embeds concrete checkpoints (URL resolves, dimensions set, poster representative). Not a 5 because there is no explicit re-validation loop after fixing (e.g., 're-run the checks to confirm'), leaving a minor checkpoint gap; it is clearly above a 3 since the sequence and criteria are well defined.

Progressive Disclosure

The body is a concise overview with clear sections, and it points to a single one-level-deep reference that exists and is clearly signaled: "For full implementation details, code examples, and framework-specific guidance, see `references/rule.md`" (verified present in the bundle). Content is appropriately split — full code examples and CLS CSS live in the reference — matching the ideal anchor exactly.

Specificity

The description names the domain ("the `poster` attribute specifies the URL of an image to show as a placeholder before the video is loaded or played") but lists only one concrete behavior; it is descriptive rather than action-listing, matching the 'names domain and 1-2 concrete actions, not comprehensive' anchor. It is not a 2 because it does state a specific concrete behavior, and not a 4 because it never enumerates several actions (check, add, validate).

Completeness

The 'what' is clear (poster attribute = URL of placeholder image), but the 'when' clause is garbled: "Use when applies to all `<video>` elements in HTML" is grammatically broken, leaving the trigger only weakly and confusingly expressed. This fits the 'clear what but when missing or only weakly implied' anchor; the broken clause prevents a 4.

Trigger Term Quality

Relevant keywords are present ("`<video>` elements in HTML", "`poster` attribute", "CLS (Cumulative Layout Shift)") but common natural variations are missing — notably "thumbnail" (the skill's own name), "video preview", or "placeholder image" phrasing users would say. Not a 4 because the natural synonym coverage is incomplete; not a 2 because the terms given are domain-specific and genuinely matchy.

Distinctiveness Conflict Risk

"This is distinct from captions or audio descriptions, which are accessibility requirements" explicitly disambiguates adjacent skills, and the video-poster niche is narrow. Minor overlap risk remains with general video/accessibility or CLS/layout skills, so it does not reach the fully distinct anchor at 5.