Property border-radius doesn't exist in CSS level 2.1 but exists in : 6px 6px

Question

Hello,

I am new in web designing. I have some probuelms in my website. Firstly That is not cross browser website and I want to make it. But I don't know how to do this. I have read some articals about this but have not got any help. Please tell me how I can make cross browser website. Secondaly I have another problem. When I have validate my website css file then I have got these errors 218 .box Property border-radius doesn't exist in CSS level 2.1 but exists in : 6px 6px 219 .box Property -moz-border-radius doesn't exist : 6px 6px 220 .box Property -webkit-border-radius doesn't exist : 6px 6px. But I don't know how to solve this also. Please tell me is anyone know about that.

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