Write base utilities for small screens; layer tu-md-* and tu-lg-* inside media queries in your stylesheet. Unprefixed classes apply everywhere unless overridden.
Going deeper
In production utility-first CSS work, Mobile-first utilities matters when documents, stylesheets, or apps must stay maintainable across teams and releases—not only in isolated demos.
Common pitfalls
Watch for copy-paste configs, skipping validation or tests, and mixing concerns (structure vs presentation vs behavior) in one layer.
Practice
- Apply one technique from this lesson in the playground.
- Write one interview-style sentence explaining when you would use mobile-first utilities on a real project.