How do I center a div with flexbox?

Set the parent to display: flex; align-items: center; justify-content: center;. That centers the child on both the horizontal and vertical axes regardless of its size.

What is the difference between justify-content and align-items?

justify-content aligns items along the main axis (the direction set by flex-direction), while align-items aligns them along the cross axis (perpendicular to it).

Is anything I search or copy uploaded?

No. The whole cheatsheet and its search run in your browser, so nothing you type or copy is ever sent anywhere.

CSS Flexbox Cheatsheet

CSS flexbox cheatsheet — searchable reference

Every flexbox property you actually use, grouped so you can find it fast: container setup, main-axis alignment (justify-content), cross-axis alignment (align-items), item sizing (grow, shrink, basis) and a set of common recipes like perfect centering and responsive card rows.

Search by property name or by what you want to do — “center”, “wrap”, “grow” — and copy the exact CSS into your stylesheet. Everything runs in your browser; nothing is uploaded.