A finished interface is mostly invisible work: text that renders crisply, numbers that don't jiggle, hover states that don't shift layout, focus rings that don't clash, hit areas you never miss. No single item is noticeable on its own; their absence is. Run this as a pass over every component after it works and before review-checklist gates it. If the pass finds the structure itself is wrong, escalate to feeling-right.

The pass

Antialiased smoothing once at the root; fonts subsetted.
Never change font weight on hover, active, or selected. Weight reflows the text. Hold weight; signal with color.
tabular-nums on anything that changes — counters, prices, timers.
No layout shift from dynamic content. Skeletons, placeholders, and images carry the dimensions (or aspect-ratio) of the loaded state.
Real typographic characters and text-wrap: balance on headings — see line-behavior.
Interaction states that stay put. Hover transitions only the properties that change, ~150ms ease-out. Press is scale(0.96), never below 0.95. Focus rings stay neutral (grey, black, white) and never disappear. ::selection, if styled, stays a legible tint. will-change only for transform/opacity/filter, only after observing a first-frame hitch.
A fixed z-index scale (--z-dropdown: 100, --z-modal: 200, --z-tooltip: 300, --z-toast: 400) — or avoid it with isolation: isolate so children can't leak above unrelated UI. DOM order first, then one global file of sequential layers (z-index-strategy).
Hit areas ≥ 44×44 touch / 40×40 desktop, extended with a pseudo-element, never overlapping — see touch-and-focus.
Align by eye. Trailing-icon padding = text-side padding − 2px; play triangles nudge 1–2px toward the point; asymmetric SVGs are fixed in the viewBox.
Native scrollbar on the page. Customize only inside small scrollers (code blocks): 8px, translucent rounded thumb.
Empty states teach: headline, one line, primary action — sized to the filled state so completing the action doesn't shift the layout. See empty-loading-states.
Decorative elements are inert: pointer-events: none, user-select: none.
No flash on refresh. Persisted interactive state is set before first paint, never default-then-correct.
Anchors clear the sticky header (scroll-margin-top); fixed bottom chrome pads env(safe-area-inset-bottom).

Frequent offenders

Problem	Remedy
Bold-on-hover tabs shift layout	Constant weight, color signals state
Digits jiggle	`font-variant-numeric: tabular-nums`
Content jumps as images load	Explicit dimensions or `aspect-ratio`
`scale(0.9)` on press	`scale(0.96)`
Brand-colored focus ring	Neutral ring
`z-index: 9999`	Scale, or `isolation: isolate`
20px icon with a 20px hit area	Pseudo-element to 44px
Custom page scrollbar	Native; customize only small scrollers
Blank div for an empty list	Headline + line + action, sized like the filled state
Theme flashes on refresh	Persist + set before render

When to apply

The build works and now needs the last ten percent; "feels unfinished" with no single dimension obviously wrong; before any review. disambiguation routes looks right to feeling-right, feels finished here, should ship to review-checklist.

Gotcha

Polish applied to the wrong structure is thrown away with the structure. If hierarchy or spacing is wrong, this pass is premature; fix the layout, then polish.

Sources

Emil Kowalski's design-engineering practice on polish, distilled by HKTITAN.
Related: review-checklist, responsive-feedback, touch-and-focus, line-behavior, depth-and-nesting.