<Li> A single heading element (h) was added . The level of these headings was determined by the depth of the nesting . This would have allowed the use of headings to be infinite, rather than limiting use to six levels deep . </Li> <Li> The remaining presentational elements i, b and tt, still allowed in XHTML 1. x (even Strict), were to be absent from XHTML 2.0 . The only somewhat presentational elements remaining were to be sup and sub for superscript and subscript respectively, because they have significant non-presentational uses and are required by certain languages . All other tags were meant to be semantic instead (e. g . strong for strong emphasis) while allowing the user agent to control the presentation of elements via CSS (e.g. rendered as boldface text in most visual browsers, but possibly rendered with changes of tone in a text - to - speech reader, larger + italic font per rules in a user - end stylesheet, etc .). </Li> <Li> The addition of RDF triple with the property and about attributes to facilitate the conversion from XHTML to RDF / XML . </Li> <P> HTML5 initially grew independently of the W3C, through a loose group of browser manufacturers and other interested parties calling themselves the WHATWG, or Web Hypertext Application Technology Working Group . The key motive of the group was to create a platform for dynamic web applications; they considered XHTML 2.0 to be too document - centric, and not suitable for the creation of internet forum sites or online shops . </P>

​which version of extensible hypertext markup language (xhtml) is not backward compatible with html