What is a Cascading Style Sheet? I've been working with CSS for over 20 years now and to say it's one singular thing is to ignore how quickly it morphs into something vastly more powerful as the years go on.
Ultimately, what's most important about CSS is the ability to stylize sites for all manner of deivces. It's also pretty handy that that deeper you get into CSS the easier it can make your job.
CSS and HTML are easily one of the most important aspects of becoming a better web designer and developer. Liquid developers with minimal CSS/HTML knowledge take note too - the two can be deeply integrated and optimize your development cycles.
Don't take my word for it, see what Shopify has to say.
