The Perfectionist project is no longer maintained.
Beautify CSS and SCSS with Perfectionist
Issues with the output should be reported on the Perfectionist issue tracker.
$ apm install perfectionist
Or, Settings → Install → Search for
Open the Command Palette and type
beautify css or
You can also select some CSS or SCSS in a HTML file and do the above.
Format On Save option and more in the settings.
Set the keyboard shortcut you want in your keymap:
'atom-text-editor': 'cmd-shift-x': 'perfectionist:beautify-css'
MIT © Sindre Sorhus