W3C (World Wide Web Consortium) Standards

The World Wide Web Consortium (W3C) develops interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential.

http://www.w3c.org

The W3C provides the guidelines for the coding standards to use; this includes HTML, XHTML and CSS.

Here you can view the various specifications:
CSS Specifications for CSS levels 1, 2, and 3
HTML and XHTML Specifications

To ensure your skins are coded to a high standard, check your skin files against the W3C Validation tools.

Validation Tools

Validate your HTML code at W3C
WDG HTML validator
Validate your CSS code at W3C

If you have the web developers toolbar you can quickly and easily check your code from an icon within the toolbar. (This works for both localhost installations and live websites).