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web/ (contains the compiled css and javascript files build by grunt or symfony command)
YARN
You can find all important things to know about Yarn on following website: https://yarnpkg.com.
Yarn is a different CLI client for the npm registry.
To read about why we use yarn instead of npm see https://www.sitepoint.com/yarn-vs-npm/.
SCSS
You can find all important things to know about SCSS on following website: http://sass-lang.com.
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