postcss/autoprefixer ? reverse-engineered prompt

Build me a CSS prefixer tool that takes normal CSS and adds the right vendor prefixes automatically based on the browsers I want to support.

I want it to work like a PostCSS plugin, and also be usable from the command line, so I can run it on a file or wire it into my build. It should read my browser targets from Browserslist if I have that set up, and only add prefixes that are actually needed. For example, things like placeholder selectors, stretch widths, flexbox and grid related prefixes should get handled when required.

Please make it smart about modern browser support, and don’t add extra junk when it isn’t needed. If there are edge cases with old grid support, keep that behind an option or control comment like the README describes. If you need to check current behavior or browser data, look up the latest docs online.

I’d like clear tests too, since this is the kind of tool where small CSS changes matter.