To be honest, the DNN skinning community in general is not that concerned about XHTML compliant skins.
For most other (open source) CMS systems XHTML is default.
Although not all DNN core modules are fully XHTML compliant yet, DNN 5.0 is a lot more compliant then previous versions.
I know there is a group of DNN skinners that don't really care about XHTML, but we do and with us a lot of users / governements.
DNN 5.0 introduces a new Host setting, for the Default Skin Doctype.
This will set the doctype of the page if no doctype was supplied in the skin (using the SkinName.doctype.xml file or server side code).
The advantage is that you don't have to set the doctype in the skin anymore, but you can set it through the administrative interface.
We even discussed making this the default, but decided not to since some legacy skins will break using an XHTML doctype.
There are two main reasons for this.
1. IE behaves a lot nicer with an XHTML doctype and all the IE hacks needed for HTML 4.0 will still be there while not needed.
2. CSS classes in XHTML are Case Sensitive (this has nothing to do with DNN, it's a W3C standard)
This means any user can now switch the doctype even using a legacy skin, this potentially means you can expect support issues for your skins if they don't work well with an XHTML doctype. This will become more important if the install base of DNN 5 increases.
There are two possible solutions:
1. Convert your skins to XHTML
This makes the CSS much cleaner and less complex since you need a lot less CSS hacks.
(I'm not talking about Tableless here, there is no real relationship between XHTML and Tables for layout)
XHTML is more professional.
2. Set the doctype of the skin to HTML 4 transitional
Advantage: less work
And please, don't complain about Modules not being XHTML compliant (I'll do that for you ;-).
If skinners wait for the modules to become XHTML compliant and the modules wait for the skins, nothing is going to change...
We must move forward!