To maintain interoperability in the Web environment it is necessary to comply with Web standards. Current specifications of HTML and XHTML languages define conformance conditions both in specification prose and in a formalized way utilizing DTD. Unfortunately DTD is a very limited schema language and can not express many constraints that are specified in the free text parts of the specification. This means that a page which validates against DTD is not necessarily conforming to the specification. In this article we analyze features of modern schema languages that can improve validation of Web pages by covering more (X)HTML language constraints then DTD. Our schemas use combination of RELAX NG and Schematron to check not only the structure of the Web pages, but also datatypes of attributes and elements, more complex relations between elements and some WCAG checkpoints. A modular approach for schema composition is presented together with usage examples, including sample schemas for various compound documents (e.g. XHTML combined with MathML and SVG).The second part of this article contains description of Relaxed validator application we have developed. Relaxed is an extensible and powerful validation engine offering a convenient Web interface, a Web-service API, Java API and command-line interface. Combined with our RELAX NG + Schematron schemas, Relaxed offers very valuable validation results that surpass W3C validator in many aspects.