Prefix CSS and SCSS with Autoprefixer
$ apm install autoprefixer
Or, Settings → Install → Search for autoprefixer
Open the Command Palette and type Autoprefixer: Prefix.

In an HTML file, select the CSS, open the Command Palette, and type Autoprefixer: Prefix.

Open the Command Palette and type Autoprefixer: Remove Prefixes.

In an HTML file, select the CSS, open the Command Palette, and type Autoprefixer: Remove Prefixes.

There's a Run on Save option and more in the settings.
Set the keyboard shortcut you want in your keymap:
'atom-text-editor':
'cmd-shift-x': 'autoprefixer:prefix'
'cmd-shift-u': 'autoprefixer:remove-prefixes'
MIT © Sindre Sorhus