https://techcrunch.com/2017/09/25/apple-starts-collecting-browsing-data-in-safari-using-its-differential-privacy-tech/

Today’s public release of macOS High Sierra brings with it some key updates to Safari — including the ability to disable cross-site cookie tracking and turn off autoplaying ads. Arriving alongside those features is a less publicized new addition to Apple’s proprietary browser: data collection. The company is using its newly implemented differential privacy technology to gather information from user habits that will help it identify problematic websites.

This form of data collection is the first of its kind for Safari, aimed at identifying sites that use excessive power and crash the browser by monopolizing too much memory. Apple is also documenting the popularity of these problematic domains, in order to prioritize which sites it addresses first.