These are the base CSS files I use to start new projects.
Import all the files you need. In create-react-app you would do: import "jonikorpi-base-files/reset.css"; and so on.
Import this first if you're using it. It's a combination of normalize.css and the star selector CSS reset. Tries to set sane, non-annoying defaults.
Sets up a 1rem baseline grid. Meaning 1rem will always be equal to <body>'s line-height. So for example if you need a padding of 2 baselines anywhere in your CSS you can just do padding: 2rem;, and not have to worry about any of the maths involved in traditional baseline grid implementations.
The baseline grid also scales slightly, or "flexes", meaning everything measured in rems becomes slightly bigger in larger viewports. You can adjust this by overriding (see below) --baselineScaling: 0.3. (Set to 0 to disable.) Details on the technique: http://codepen.io/CrocoDillon/pen/jgmwt?editors=0010
You should override the default properties with your own, in your own CSS file: html { --baselineSize: 1em; --baselineScaling: 0.3; }.
Then just use rems in your own CSS to set up your typography, margins, paddings, heights, widths, everything: body { font-size: 0.75rem; } h1 { font-size: 1.25rem; line-height: 1.5rem; }. (Don't change <html>'s font-size or everything will break).
A few minimalistic classes that should handle most of the paddings and margins you'll need.
If you have a container and need margins between its children, use .margins-x or .margins-y. (Using both at once to create margins in a grid-like layout will not work well. Better to use CSS Grid for that.)
On their own they'll create margins equal to 1rem: <div class="container margins-x">…</div>. To adjust the size override their respective CSS variables: <div class="container margins-y" style="--margins-y: 1rem;">…</div>.
You can use .margin and .padding similarly, to quickly give a single element some padding or margin: <div class="padding" style="--padding: 2rem;">…</div>;
A couple of flexbox utility classes so that you don't have to keep writing them again and again.
reset.css destroys many default form element styles. This brings some of them back: bordered input fields and selects, slightly rounded buttons. (If needed, use .fake-link to revert the styles and make a button look like a link.)
Classes that add margins equal to env(safe-area-inset-right) and so on, for implementing easy support of iPhone X and other non-rectangular devices.
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent