ElementalCSS is just 43.4kb minified and 7.4kb gzipped. No need to use PurgeCSS or similar.
No build process. ElementalCSS is so small that it's not necessary, but the files are there if you want them.
Include the CSS and get designing. The default classless styles make your page look great, and utility classes add extra style.
Many settings can be changed without touching the CSS by using CSS Custom Properties.
Built in support for a dark theme that 'just works', and looks beautiful (this site uses the default colours).
Different CSS files are available so you can use the smaller classless styles on their own, or use the full package.