The only option to validate CSS in Dreamwaver is to validate them against different browsers it is not the same as the http://jigsaw.w3.org/css-validator/. If you want to try it just open your CSS file and click “Check Page” on your “Document Toolbar”.
2. if ican't validate CSS in dreamweaver ,is there any othe free offline method to validate css?
You can give a try to these two if you like.
http://www.blogsdna.com/1074/download-free-css-toolbox-12-a-css-validator-formatter-compressor.htm
http://www.bestfreewaredownload.com/freeware/t-free-cse-html-validator-lite-freeware-tgrvdihr.html