CSS Shared First

Core Philosophy

Shared First is a CSS authoring methodology that eliminates cascade bleed between breakpoints. The rule is simple:

Only write CSS outside of media queries if it applies to all viewport sizes. Everything else belongs inside a bounded media query that targets exactly the viewport range it is meant for.

This is in contrast to the traditional Mobile First approach, which writes base styles for small viewports and progressively overrides them for larger ones — a pattern that is error-prone and produces noisy DevTools output full of struck-through declarations.

The Two Downsides of Mobile First