I was validating CSS for a site and I it showed that it wasn't valid. Why is that?

Because it is not a standard reference to a CSS property.

It is a Mozilla specific property used for testing the functioning of what will become the CSS3 border-radius property.

Yeah it's not supported yet by all browsers, I think Chrome, Safari and FireFox support the use of some CSS3 stuff, but it's not a standard property of the CSS script yet.

Hopefully it'll be in the near future all browsers open up to using it and many more.

Have a look at some other stuff probably not supported, but it just shows you what CSS is capable of; http://www.css3.info

Originally Posted by Kacy

Yeah it's not supported yet by all browsers, I think Chrome, Safari and FireFox support the use of some CSS3 stuff, but it's not a standard property of the CSS script yet.

Hopefully it'll be in the near future all browsers open up to using it and many more.

Have a look at some other stuff probably not supported, but it just shows you what CSS is capable of; http://www.css3.info

I believe IE8 also supports it, it will be great once all major browsers do. Then we don't have to waste time creating rounded border images.

Originally Posted by Kacy

Yeah it's not supported yet by all browsers, I think Chrome, Safari and FireFox support the use of some CSS3 stuff, but it's not a standard property of the CSS script yet.

Hopefully it'll be in the near future all browsers open up to using it and many more.

Have a look at some other stuff probably not supported, but it just shows you what CSS is capable of; http://www.css3.info

The property the OP asked about "-moz-border-radius" will NEVER be supported by "all browsers"
It is a MOZILLA specific property, just as "webkit-border-radius" is not standard and is specifically for the webkit browser engine (Apple's Safari and Google's Chrome browsers)