Conciseness

The body is lean and imperative throughout — "Place one progress line on top with transform-origin: top", "Collapse to a simple left rail on small screens" — with zero conceptual padding (no explanation of what IntersectionObserver or prefers-reduced-motion is) and every section earning its tokens. It matches the 5 anchor "lean and efficient; assumes Claude's competence" and sits above 4 because there are no instances of over-explanation to trim.

Actionability

Guidance is mostly executable: the steps data structure, the normalized-progress formula with scaleY transform, specific ARIA usage ("aria-current=\"step\""), and concrete viewport widths (390/768/1024/1440). It falls short of fully copy-paste-ready because the code snippet's inputs (viewportAnchor, lineStart, lineEnd) are never defined and no listener wiring is shown, leaving minor gaps; it is above 3 because the code is real, not pseudocode, and the layout decision rules are concrete and testable.

Workflow Clarity

Sections form a clear build sequence (model → line → layout → animate → navigation → reduced motion → verify) and the Verify section is an explicit checklist ending with an acceptance criterion ("The active step and line head must agree at every boundary"). It does not reach 5 because there is no error-recovery/feedback loop (what to do when a checkpoint fails), and it stays above 3 because validation is not merely implied — it has its own dedicated checklist section.

Progressive Disclosure

The body is well organized into seven focused sections, but the closing pointers — "Use [demo/index.html](demo/index.html) as the working reference and [demo/PROMPT.md](demo/PROMPT.md) to recreate or remix it. Keep [REFERENCES.md](REFERENCES.md) as the links-only implementation source list" — reference files that do not exist anywhere in the bundle (no references/, scripts/, assets/, or demo/ directories are present). Scored against the actual bundle structure per the judging guidelines, dangling references leave navigation broken at the one place the skill points outward, which fits the 3 anchor (structure present, references not usable) rather than 4.

Specificity

The description enumerates concrete capabilities — "data-driven vertical or horizontal scroll story with a base line, progress fill, active step states, responsive collapse, semantic fallback, and reduced-motion behavior" — listing multiple specific actions with comprehensive coverage, matching the top anchor. It is not below-5 material because no capability area of the skill is left unnamed.

Completeness

Both halves are explicit: the "what" is the concrete capability list in the first sentence, and the "when" is the explicit "Use for onboarding, checkout, roadmaps, recipes..." clause with concrete trigger phrases. This matches the 5 anchor exemplar structurally and in concreteness; a 4 would require the when-clause to be less explicit or specific, which it is not.

Trigger Term Quality

Natural scenario terms are well covered ("onboarding, checkout, roadmaps, recipes, case studies, service processes, histories, or narratives"), but common phrasings users would actually say — "timeline", "steps", "stepper", "scroll animation" — are absent from the description itself ("timeline" appears only in the name, which the description judge does not score). Good coverage with a few natural terms missing fits the 4 anchor; it exceeds 3 because multiple real user scenarios are named, and falls short of 5 because direct synonyms for the core artifact are missing.

Distinctiveness Conflict Risk

The description carves a clear niche — scroll-linked progress visualization of ordered sequences — with scenario triggers (onboarding, checkout, roadmaps) that are unlikely to fire for unrelated skills, matching the 5 anchor's "clear niche with distinct triggers; minimal conflict risk". It does not fit the 4 anchor because there is no meaningful overlap with a closely related generic skill given the specificity of the trigger list.