1. XHTML2 and HTML5: same in both

html, head, body, title, meta, link, style, script,

p, pre, blockquote, div, h1-h6, cite

ol, ul, li, dl, dd, dt

table, td, th, tr, caption,
col, colgroup, tbody, tfoot, thead

a, area, abbr, acronym, code, dfn, em, kbd, samp, span, strong, sub, sup, var

img, object, param

2. Not in either

big, tt

dir (deprecated)

basefont (deprecated)

frame, frameset, noframes

center (deprecated)

font, s, strike, u (deprecated)

applet (deprecated)

isindex (deprecated)

3. Not in HTML5

ruby, rbc, rtc, rb, rt, rp

4. Not in XHTML2

b, i, small

noscript

5. Renamed/remodelled in XHTML2

base → xml:base
bdo: assumed in dir attribute
br → line element
ins, del: as attributes edit="ins"
hr → separator
map → nl
q → quote

6. Reintroduced in HTML5

iframe

legend (deprecated)

menu (deprecated)

7. Forms

In both: input, label, select, textarea

Renamed in XHTML2:
optgroup → choices
option→ item
fieldset→ group
button→ trigger

8. Added to both XHTML2 and HTML5

section, output

9. Added to XHTML2

h, blockcode, access, handler

10. Added to HTML5

progress, article, header, nav, aside, footer, meter, dialog, time, figure, video, audio, source, canvas, datalist, event-source, details, datagrid, command, embed, m

11. Attributes

Many differences here

12. Why the XML serialization of HTML5 should not be called XHTML5

1. It would cause confusion (for instance, it is not the 5th version of XHTML, which is what it suggests given the other numberings of XHTML, but the XML version of HTML5).

2. Since XHTML refers to a family of modularized languages, and HTML5 does not use this framework, it would mislead.

3. There is no need to have two names for HTML5. Just talk about the XML serialization of HTML5 if the need arises.