GRUNT install on OSX

First off, make sure you installed yarn like described here.

In your Terminal app navigate to the folder that you cloned the TYPO3 source into. Switch into the Build folder and run

yarn install

This will install Grunt as well a s couple of Task plugins we use in the TYPO3 core. To make sure grunt is running, run

grunt css

This will run all registered tasks defined in the TYPO3 core.