Conciseness

The body is short overall, but the soft-404 point is repeated three times (Quick Reference: 'Return a real HTTP 404 so the page is not treated as a soft 404'; Check: 'Confirm the route returns HTTP 404'; Fix: 'returns status code 404 so the page does not become a soft 404'), and Quick Reference largely duplicates Check/Fix. It is not a 2 because there is no padded prose, but anchor 4 is too generous given the clear tightening available.

Actionability

Concrete, directive guidance is present ('Confirm the route returns HTTP 404 and not a branded page with a 200 response', 'Flag exact elements, attributes, and routes where the rendered HTML violates the rule'), with implementation detail appropriately deferred to references/rule.md, which contains full HTML/Next.js/Nginx code examples and Verification steps. Not a 5 because the body itself lacks any executable check command (e.g., a curl status-code test) for the common case.

Workflow Clarity

This is a simple single-purpose review skill and the single action — verify a custom 404 page exists with helpful navigation and returns HTTP 404, flagging violations — is unambiguous across the Check, Fix, and Code Review sections. The simple-skill exception applies and no validation cap is relevant since the skill is non-destructive.

Progressive Disclosure

The body is under 50 lines, organized into clear sections, and points to a single one-level-deep reference ('For full implementation details, code examples, and framework-specific guidance, see references/rule.md'), which exists as a real, comprehensive file with no further nesting.

Specificity

The description names the domain ('Create a custom 404 error page') and lists several specific review targets ('reviewing templates, rendered HTML, or shared components') plus a concrete action ('Validate the final browser-facing markup'). It falls short of a 5 because it only states review/validate actions — no fix or create actions — so coverage has minor gaps rather than being comprehensive.

Completeness

Both parts are explicitly present: 'when' via 'Use when reviewing templates, rendered HTML, or shared components related to Create a custom 404 error page' and 'what' via 'Validate the final browser-facing markup, not just the source framework abstraction'. Not a 5 because the 'what' is narrow (validation only) while the skill body also covers fixing and explaining.

Trigger Term Quality

It includes natural terms users would say ('404 error page', 'templates', 'rendered HTML', 'shared components'), giving good keyword coverage. It is not a 5 because common synonyms like 'Page Not Found', 'not found page', or '404 handler' are missing.

Distinctiveness Conflict Risk

'Create a custom 404 error page' is a clear niche with distinct trigger terms (404, rendered HTML templates, shared components) and minimal overlap risk with other skills.