Although CSS has seen the light of the world [more than 20 years ago][1], it's still surprisingly under-researched. Our websites keep breaking in spectacular ways: visual inconsistencies caused by seemingly minor changes can go undetected for days, negatively impacting the user experience (UX), inducing customer churn and frustration, which can lead to lost opportunities. All too often, CSS is not perceived as a critical resource requiring proactive and deliberate care, but as an annoying necessity to advertise the brand to the consumer.
CSS is a very simple language: its [parsing][2] and [matching][3] semantics are incredibly easy to understand and implement. Moreover, it is very forgiving and