For improved web design development, consider installing the CSS 3 Intellisense Schema in Visual Studio 2010:
This schema enhances CSS3 support in Visual Studio 2010, providing developers with better tools for designing websites and supporting CSS3 properties.
In addition, Service Pack 1 (SP1) offers some enhancements to CSS3 support, although not as extensive as HTML5. The editor now recognizes advanced selectors such as div:nth-child(2n+1) without causing validation errors, and new color values like rgba, hsl, hsla, and 8-digit hex values are also supported.
Read more here