When you’re working on a website your browser cache’s certain files, like CSS for example. Most people say to just press Ctrl+F5 on the HTML page in order to see the changes, but in some cases this doesn’t reflect the changes.
If you made changes to a CSS file, you might need to clear the cache of the CSS file:
Open the CSS file in your browser, and hit Ctrl+F5 there.
The reason hitting Ctrl+F5 on the HTML page does not work is because all that’s doing is clearing the cache on the HTML page, not for the CSS file.