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What version of HTML do I mark my pages up in?
Almost always XHTML 1.0 Strict. (I'm aware of course that, ideally, I ought to be marking everything up in HTML 4.01 Strict.)
When will I change the version of HTML I'm using?
As soon as FF and IE can have a good go at rendering HTML 5 [w3.org].
I went through a conversion from XHTML Strict to HTML Strict and I ended up going back to XHTML Strict because of applications being developed using XHTML and attempting to blend those with HTML pages and getting things to validate just didn't work out. So, for now, XHTML 1.0 Strict and Transitional where applicable. I'm looking forward to HTML 5 and its widespread adoption. Until then, I need to appease the developers. I'm on Windows so most everything is XHTML these days.
As for HTML5, I won't use it for non-personal sites until the final specification is published - otherwise it's a moving target.
What does Windows have to do with it?
Most of us on Windows are into .NET. If I'm not mistaken, all .NET application development software outputs XHTML. I've battled with various programmers over the years to write HTML 4.01 Strict to no avail. So, I gave in and now write XHTML 1.0 Strict by default.