If all your syntax seems fine, then its most likely a browser cache problem that we can easily fix. In your html/php file, reference your new .css style sheet (e.g. styles.css) by adding an extra random parameter. This will force browsers visiting your page to fetch your latest styles.css.
In the of your html/php file, you should have something like this to load your new styles.css file:
<link rel="stylesheet" type="text/css" href="styles.css" />
simply change it to be like this:
<link rel="stylesheet" type="text/css" href="styles.css?ref=v1" />
that extra "?ref=v1" will prevent browsers from re-using the styles.css file they have cached, and will force browsers to get your very latest styles.css file. As you make updates to the styles.css file and upload them to your web server, just change the "v1" to "v2" etc. or whatever naming system you like so that browsers are forced to reload the latest styles.css. Note adding this "?ref=v1" to the link does not need you to change the name of your styles.css file (you can change the file name but i find that gets messy). This is a simple and clean way to force browsers into re-fetching your very latest .css file.