Design system, content, coherence

10.1 Tokens: style lives in one place

A design system starts with tokens, the named variables for colors, spacing, type. Change the token, the whole product changes with it. Hard-code a color in one screen and you've started a slow drift no one will fully fix. Consistency isn't discipline applied a thousand times, it's one source applied everywhere.

10.2 The one invariant that keeps a product coherent

Pick a small, fixed palette and hold it. A handful of colors used with meaning (one for the primary action, one for danger, one for success) is what makes a product feel like one product. The failure mode isn't ugliness, it's ten slightly different blues that each felt fine alone.

10.3 The component registry

Reuse before you recreate. Keep a registry of the building blocks (the button, the card, the input) and use them. Every one-off component is future inconsistency and future maintenance. When a file grows past a couple hundred lines, that's the signal to split it, not a nag.

10.4 Content is calibrated on a corpus, not decreed

Your product's voice (the microcopy, the empty states, the error messages) isn't invented per screen. It's calibrated on a reference: a few examples that define the tone, and everything else matches them. Content decided screen by screen ends up sounding like five different people, because it was written by five different moods.

10.5 Performance and accessibility (the light touch every builder owes)

Two things you don't have to master, but must not ignore. Performance: a slow product feels broken even when it works. Watch the basics (how fast the first real content appears, how quickly the page becomes usable) and treat a heavy page as a bug, not a detail. Accessibility: some users navigate by keyboard, some use screen readers, some can't tolerate motion. The cheap wins go a long way: visible focus states so keyboard users can see where they are, honoring the "reduce motion" setting, real text alternatives for images, enough color contrast. It's not extra polish. It's whether a chunk of your users can use the thing at all.