Web 2.0 is a generic buzzword signifying the evolution from static HTML pages to AJAX rich interactive media, it's sometimes attributed to a method of styling a page (glass effects, shiny surfaces, reflections, "bubble text", etc.) though that's more of something which became attributed to it due to an unfortunate load of trends appearing at the same time as the buzzword became created, though the majority of this was just coincidence wrongly associated to the term by people.
XHTML doesn't really have many advantages over HTML, it's not an evolution of the language... if you like strict rules which make you code properly or it'll error out you might like XHTML and
if you need XML related injection into your site it could be useful, generally though there's no real reason for most people to use XHTML over HTML, more importantly XHTML doesn't work properly in IE and XHTML 2.0 was abandoned in favour of HTML5 meaning the language is "technically" deceased.

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