You should validate your HTML and CSS using the W3 validator get it clean and then see if Safari on Mac can render it.
Check that the browser isn't replaying a cached copy of an older version of your page. In the worst case you may have to close and re-load the browser. Some browsers let you override the local cache by forcing a reload using a key combination such as Ctrl+F5. There may be something similar for Safari on Mac.
Use "View Source" to check that the browser is receiving the latest version of your page. You can explicitly fetch external CSS to see if your browser is fetching the latest version of the CSS.