Fun times ahead!! Read all about HTML 5 differences from HTML 4

The following elements are not in HTML 5 because their effect is purely presentational and therefore better handled by CSS:
* basefont
* big
* center
* font, although it is allowed when inserted by a WYSIWYG editor due to limitations in the state of the art in user interface for these editors.
* s
* strike
* tt
* u

The following elements are not in HTML 5 because their usage affected usability and accessibility for the end user in a negative way:
* frame
* frameset
* noframes

The following elements are not included because they have not been used often, created confusion or can be handled by other elements:
* acronym is not included because it has created lots of confusion. Authors are to use abbr for abbreviations.
* applet has been obsoleted in favor of object.
* isindex usage can be replaced by usage of form controls.
* dir has been obsoleted in favor of ul.

Some attributes from HTML 4 are no longer allowed in HTML 5.

If they need to have any impact on user agents for compatibility reasons it is defined how they should work in those scenarios.
* accesskey attribute on a, area, button, input, label, legend and textarea.
* rev and charset attributes on link and a.
* shape and coords attributes on a.
* longdesc attribute on img and iframe.
* target attribute on link.
* nohref attribute on area.
* profile attribute on head.
* version attribute on html.
* name attribute on map, img, object, form, iframe, a (use id instead).
* scheme attribute on meta.
* archive, classid, codebase, codetype, declare and standby attributes on object.
* valuetype and type attributes on param.
* charset and language attributes on script.
* summary attribute on table.
* headers, axis and abbr attributes on td and th.
* scope attribute on td.

In addition, HTML 5 has none of the presentational attributes that were in HTML 4 as they are better handled by CSS:
* align attribute on caption, iframe, img, input, object, legend, table, hr, div, h1, h2, h3, h4, h5, h6, p, col, colgroup, tbody, td, tfoot, th, thead, tr and body.
* alink, link, text and vlink attributes on body.
* background attribute on body.
* bgcolor attribute on table, tr, td, th and body.
* border attribute on table, img and object.
* cellpadding and cellspacing attributes on table.
* char and charoff attributes on col, colgroup, tbody, td, tfoot, th, thead and tr.
* clear attribute on br.
* compact attribute on dl, menu, ol and ul.
* frame attribute on table.
* frameborder attribute on iframe.
* height attribute on iframe, td and th.
* hspace and vspace attributes on img and object.
* marginheight and marginwidth attributes on iframe.
* noshade attribute on hr.
* nowrap attribute on td and th.
* rules attribute on table.
* scrolling attribute on iframe.
* size attribute on hr, input and select.
* style attribute on all elements with the exception of font.
* type attribute on li, ol and ul.
* valign attribute on col, colgroup, tbody, td, tfoot, th, thead and tr.
* width attribute on hr, table, td, th, col, colgroup, iframe and pre.