drhowarddrfine - 3:28 pm on Jan 9, 2013 (gmt 0)
CSS is a generic term for every version of 'cascading style sheets'. CSS3 is the latest version. Whether you use properties from CSS3 or not depends only on whether the browser can render them. Whether the browser can render them depends on which browser version you are using.
Rounded corners should work in all current browser versions. Older versions may need 'vendor specific prefixes' to work, such as -moz-border-radius, -webkit-border-radius, etc. IE versions before IE9 don't work with border-radius at all.
All that said, I don't know anything about how WordPress works.