Changes from HTML4
- HTML5 specifies its own syntax rules authors have to follow. These syntax rules are compatible with the XHTML syntax rules althoug this does not imply that parsing such a document with an HTML parser will give the same result as parsing it with an XML parser.
- HTML5 defines its own parsing rules (including "error correction") for text/html resources and no longer assumes SGML features are supported.
- datalist (Web Forms 2)
- output (Web Forms 2)
These elements have a new meaning in HTML5 which is incompatible with HTML4. The new meaning better reflects the way they are used on the web or gives them a purpose so people can start using them.
That these elements are dropped means that authors are no longer allowed to use them. User agents will still have to support them and HTML5 will probably get a rendering section in due course that says exactly how. (isindex for instance is already supported by the parser.)
- acronym (use abbr instead)
- applet (use object instead)
- noscript (only dropped in XHTML5)