System Drupal 7 becomes deprecated, so I working on migration of all my web sites to newer Drupal 8. Now I managed to upgrade the web of housing association made in the year 2015. There is a lot of incompatibilities in the new Drupal version, so upgrading it means to create also a new theme and replace few deprecated modules.
This language tells the browser how a website should look like. It defines colors, sizes, margins, fonts (etc.) and applies them to selected elements on web page.