Since version 1.4 all vendor prefixes were removed from SCSS files and Autoprefixer is used instead. Now after compilation CSS styles are passed through Autoprefixer to add necessary vendor prefixes. As you know vendor prefixes are important to ensure cross-browser compatibility of latets CSS3 features.
Autoprefixer is integral part of Grunt building process. So to customize its settings you need to go to gruntfile.js and found autoprefixer task. You will see browsers settings under options. It's currently set to ['> 5%', 'last 3 versions', 'ie 10', 'ie 11'] which reflects the browser set I retrieved from my Google analytics. Generally it's a good practice to collect some analytics about what Browsers/OSs your visitors use. And then customize Autoprefixer settings based on this data.
For more information about available Browserlist options visit https://github.com/ai/browserslist.
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