EarthWeb's JavaScripts portal. Each script available to download.
The World Wide Web Consortium (W3C) develops interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential as a forum for information, commerce, communication, and collective understanding. They have information about HTML, XML, XSL, CSS, etc.
The W3C HTML Validation Service; a free service that checks documents like HTML and XHTML for conformance to W3C Recommendations and other standards.
The W3C CSS Validation Service; a free service that checks Cascading Style Sheets (CSS) in (X)HTML documents or standalone for conformance to W3C recommendations.