Lean throughout — it assumes CSS competence, never explains basic concepts, and every section (Rules, Base Tokens, code blocks, Taste Rules, Quick Checks) earns its tokens with no padding.

Provides fully executable, copy-paste-ready CSS for tokens, container-line pseudo-elements, corner squares with matching HTML, and a content wrapper — no pseudocode or missing details.

A single-purpose skill whose action is unambiguous: the Rules sequence the approach and the Quick Checks section acts as an explicit verification checklist, satisfying the simple-skill allowance.

A cohesive single-purpose pattern with no need for external references; content is well-organized into clearly labeled sections with no nested or dangling references.

Lists multiple concrete actions — 'Add vertical container-size guide lines with mini corner squares' — naming two distinct, specific operations rather than vague abstractions.

Clearly answers both what ('Add vertical container-size guide lines with mini corner squares...') and when via an explicit 'Use when' clause with multiple triggers.

'Use when asked for container lines, measured layout guides, vertical boundary lines, editorial grid markers, or small corner-square frame details' gives good coverage of natural phrases a user would actually say.

Targets a narrow, distinctive niche (container guide lines plus corner squares for web layouts) with specific triggers unlikely to fire for unrelated skills.