CSS has been around for years. They are used for document design as a technical means of ensuring that the designs of Print Designers are printed precisely to specification. In relation to web design, cascading style sheets
serve the same purpose with the additional functionality of instructing the web browser how to read the document being looked at. The cascade in website sheets is the vital component as it guides the web style sheets to where it wants them to go. These sheets are a default setting that the web browser uses to display a page if no further specific instructions are embedded. The browser needs to know which instructions supersede preferentially.
CSS can also be used to define how XML including XHTML mark up, appears in your Magento 2 web store. You can also use print style sheets to dictate the print format of the web page and other style sheets can be used to show the web page for a slide show or on a projector.
Professionally written style sheets can be updated to quickly permit adjustments to what you want to prioritise without having to change the underpinning XHTML.
Learning and implementing CSS is a valuable asset to any designer as it increases the options to do more with a website. It is worthwhile foregoing the difficulty in learning CSS as the results are quite phenomenal.