Exact spacing capture: arbitrary values (pt-[40px]) instead of renumbering the scale to Tailwind

The question: the spacing scale (unysonplus_default_spacing_scale()) has no 32px or 40px step — the range 24→48px is one gap — so the Site Converter, which snaps a source's captured padding to the nearest slug, rendered e.g. a 40px bottom padding as 48px (up to 12px error). Should we renumber the whole scale to Tailwind numbering (pt-8=32px, pt-10=40px…) to fill the gap, or keep the scale and add arbitrary values (pt-[40px])?

Context

UnysonPlus is a Bootstrap-5 framework. The scale's 0–5 slugs deliberately equal Bootstrap's $spacers (pt-5=48px in both), and it generates its own .pt-N utilities from that scale. The 6–12 tail is a house extension. The gap surfaced converting real sites (a Wegic cupcake site whose CTA uses pb-10=40px), and it kept demos reaching for scoped CSS instead of the native option.

We prototyped both approaches in a throwaway clone of the marketing site (localhost/spacetest).

Options considered

• Renumber the scale to Tailwind (slug = px ÷ 4). Gains recognizable names and fills the gap, but every probe surfaced more breakage. It needs a one-time migration rewriting three stores — the saved spacing_scale/gap_scale presets (a saved override shadows the PHP default), the per-post builder JSON, and the site-wide default_gap trio (which the row gutter pulls via --gap-N). It diverges from Bootstrap: pt-5 becomes 20px, so pasted Bootstrap markup silently mis-renders. And it revealed a two-read-path trap (the CSS getter and the admin field read the scale from different stores). Each issue is fixable, but collectively the surface area is large — for naming.
• Keep the scale, add arbitrary values (pt-[40px], mb-md-[3.5rem]). A spacing option may hold an arbitrary token; the per-page dynamic CSS emits a matching escaped rule (.pt-\[40px\]{padding-top:40px}), breakpoint-aware. sc_sanitize_class() preserves only the strict arbitrary pattern (no XSS surface). Non-breaking — scale, containers, gutters, Bootstrap alignment, every existing pt-N untouched — and more precise than the renumber (any exact value, not just Tailwind's discrete steps). Contained to three files. The converter emits a preset slug when the captured px lands on the scale, else an exact pt-[Npx].

Decision

Keep the Bootstrap-aligned scale exactly as-is and add arbitrary spacing values. The converter now captures spacing losslessly (proven by the browser-free tailwind-matrix fixture: all 35 Tailwind steps round-trip exactly), and the pinky-bites CTA takes a true pb-[40px] (was 48px).

Why

The renumber's only real benefit over arbitrary values was Tailwind-recognizable names — and for a Bootstrap framework, Bootstrap-aligned names are the more coherent identity, not less. Arbitrary values solve the actual problem (exact capture) with zero migration and zero divergence, so the naming-only upside never justified the breakage. The clone-site trial was decisive: it caught the saved-scale shadow, the default_gap gutter dependency, and the two-read-path trap that would each have silently broken production had the renumber shipped straight to localhost/.

Status: Accepted. Follow-up: a "Custom…" branch on the spacing field (Phase 2) so custom values are editable in the builder — converter-set values already render without it.