1. Using pxPx locks your design to a fixed size for all screens.You should always try to use relative measurements such as em, rem, %, etc and wherever possible
2. Using one CSS file for styling
Using single file in big projects can create mess.
Try to use different CSS files for different components.
This will also increase the reusability of code.
3. Using inline CSSYou have to write the same code again & again for different elements, there is no place to put these styles for reuse.Always try to use external stylesheets until needed.
4. Using !important everywhereThe only thing that can override an !important is another !important, and once you go down that road, it never stops.
There's no point in jamming your CSS with a lot of !important styles and become gridlocked.
5. Not using proper naming conventionMake sure to name classes, ids and other attributes properly and in a similar manner.Try storing color values in variables instead of copying same color over and over again.
6. Not designing beforehandIt feels amazing to just start building without thinking much, but this can turn in a mistake once things get complex.
Try to have a clear picture of project.
7. Being too creativeYou don't need heavy CSS or animations forced into your website.Elegant design with good UX/UI is preferred by everyone.