Wikipedia
Standards-compliance is the compliance of a website or web browser with the web standards of the World Wide Web Consortium (W3C). To ensure interoperability a standards-compliant web site does not use proprietary software methods or features of a browser.
Although there is no perfect browser that adheres to all standards, advancement has been made by most web browsers in the past few years that will ensure better interoperability. In the past a standards-compliant browser sometimes meant a browser other than Internet Explorer (which had poor compliance prior to the release of version 8.0 in 2009).
Current use of the term "standards-compliance" generally refers to the adherence to coding practices in relation to the use of HTML or XHTML, with Cascading Style Sheets (CSS) to define the layout, colors, and fonts of a web page. The Web Standards Project (WaSP) is a group, mainly composed of experienced web developers, whose mission is to encourage the use of these standards globally. Their recent efforts have been to promote the use of and adherence to the CSS 2.0 web standard by browsers, including how browsers respond to invalid markup and styles. The tests developed by WaSP are called Acid1, Acid2, and Acid3, with each testing CSS1, CSS2, and CSS2+ (CSS2 + Client-Side Scripting), respectively.