Use this boilerplate as a starting point for your work.
A basic project-structure with the following folder and files.
assets/css/app.css 👈 main stylesheet which imports all other stylesheets, don't write css definitions there, for a cleaner organisation
assets/css/fonts.css 👈 location for including custom fonts, IBM Plex Sans Variable Font-Face is included as an example in this project
assets/css/media.css 👈 stylesheet for global styling of media elements like images
assets/css/modern-normalize.css 👈 makes browsers render all elements more consistently and in line with modern standards
assets/css/preflight.css 👈 removes all default browser margins and resets a little more
assets/css/typo.css 👈 typo styling