What is the difference between HTML 3 0 and HTML 4 0?

One difference in the latest HTML specifications lies in the distinction between the SGML-based specification and the XML-based specification. The XML-based specification is often called XHTML to clearly distinguish it from the more traditional definition; however, the root element name continues to be HTML even in the XHTML-specified HTML. The W3C intends XHTML 1.0 to be identical with HTML 4.01 except in the often stricter requirements of XML over traditional HTML. XHTML 1.0 likewise has three sub-specifications: strict, loose and frameset. The strictness of XHTML in terms of its syntax is often confused with the strictness of the strict versus the loose definitions in terms of the content rules of the specifications. The strictness of XML lies in the need to: always explicitly close elements (<h1>); and to always use quotation-marks (double " or single ') to enclose attribute values. The use of implied closing tags in HTML led to confusion for both editors and parsers.