CSS 2.1 errors: not a known property name

You're validating your stylesheets against CSS 2.1, but those properties are CSS3.

Change your stylesheet validation to CSS3.

Validation errors are all fine, because they are using the proper CSS 2.1 Documentation and your trying to validate CSS3 properties in a CSS 2.1 context.

What you probably want is to build your application bypassing the CSS Errors, no matter what they are, for that you can transform this "errors" into warnings using Visual Studio Options:

in The Visual Studio menu click on:

Tools > Options... and drill down Text Editor > CSS > Miscellaneous

and just make the CSS Errors to be treated as warnings

2013 version