XHTML stands for EXtensible Hyper Text Markup Language and it is a stricter and cleaner version of HTML recommended by W3C (World Wide Web Consortium). XHTML is the effective inheritor of HTML 4.01 and although it is almost identical to its predecessor it is aimed to replace it. There are three levels of XHTML compliance (sorted by importance in a descending order):
All Telerik ASP.NET AJAX controls are XHTML 1.1 compliant except for RadChart, which is XHTML 1.0 Transitional compliant.