Browsers all cache the stylesheets of websites for a while once you’ve visited a site (or page) once. They do this to make the pages load faster when you visit again and for published websites that aren’t changing, it’s not a problem, as the fresh content still displays well.
However, when your website is in development and style changes are being made, it means that you can open a page that has been changed and not see the changes.
Here’s where hard-refresh comes in. This forces the browser to fetch the edited stylesheet.
On a computer:
If using Chrome or Firefox, go to the page that has changed and click CMD+SHIFT+R
If using Safari, go to the page that has changed and click CMD+OPTION+E
Both these options enable you to clear the the browser cache without losing any history or data.
On an iPhone:
Go to Settings > Safari > Clear History & Website Data.
You’ll need to re-enter the url to get the page after this as this closes all your currently-open pages.