Lean and dense; assumes Claude knows Tailwind/design-system basics and devotes every section to repo-specific rules and class-string patterns rather than background explanation.

Concrete file paths (packages/playground-ui/src/ds/components/, theme.css) and copy-pasteable class patterns (bg-(--row-bg), max-w-100, max-h-[calc(100dvh-3rem)]) make the guidance directly executable; absence of full code blocks is excused for an instruction-only skill per the rubric notes.

The numbered 'Choosing a class value' decision ladder (rungs 1–5) gives a clear sequence, and the 'Review smells' section acts as an explicit verification checklist; no destructive/batch operation is involved so the cap does not apply.

Under ~50 lines, organized into seven clearly headed sections, with the only external pointer ('read the tailwind-v4 skill') one level deep and clearly signaled; no bundle files are needed or referenced, so well-organized sections suffice per the simple-skills note.

Lists multiple concrete actions — 'creating or modifying any application UI — pages, components, styling, or tokens' — naming the design system explicitly rather than gesturing vaguely.

Answers both what ('How to build Mastra frontend interfaces with the @mastra/playground-ui design system') and when ('This skill should be used when creating or modifying any application UI...'), plus an explicit out-of-scope statement about the docs site.

Covers natural terms a user would say ('pages, components, styling, or tokens', 'application UI', 'design system'); 'tokens' is slightly technical but the rest are common user phrasings.

Clear niche scoped to the @mastra/playground-ui design system with an explicit scope exclusion (docs site), making overlap with other skills unlikely.