You'd have to aim specifically for the mobile device standards to get them to display ideally on a phone etc.
And something I completely forgot about earlier. It depends why its invalid, as to if its a real problem or not. If you are missing an alt tag, its not the end of the world (matter of fact, it won't make a difference at all). Missing the DOCTYPE, and you kiss goodbye any form of cross-browser compatibility.
Well, alt tag helps in Google Image Search optimization. Anyways, one thing we can say for sure - validity won't do any harm. It isn't SO important but it's a sign of professionalism and quality. One good advantage of valid xhtml I think should be mentioned is that it can be easily converted to xml or to some other formats that are used in pdas and cellphones. Definately easier that not valid xhtml.
This is only true if the valid XHTML is also well-formed (usually is if it's valid), and follows the protocol for XML (Valid XHTML does not have to because it's served as text/html). One for-instance is character entities. You can have valid XHTML with <pre>©</pre> but it's not one of the XML "character entities" (you get only 5). Since one of the 5 is not recognised by HTML, you really only get 4 (<pre>> < & "</pre> as <pre>'</pre> is XML-only). There's some other things like this which I've run across. If you write your XHTML as if it was really XML while keeping HTML browsers in mind, then yeah, you can do it.
Having Valid and Semantic code helps in many areas. It helps search engines, flexibility, and support for screen readers.
I would have thought that the official ones would be, you could always use the W3C validator to check yours
Most are coded well because Wordpress does most of the work. Emphasise on the most, its quite easy to still do a bad job of it
Well, wordpress uses crap like target="_blank" etc and isn't all that semantic...which is why Dan Schulz is working on a "semantic WordPress" so that everyone can has cheezeburger and eat it too.
You are not supposed to decide instead of the user whether he should leave the current tab/window or not. That's not right. It's his/her own browser and he/she will open links in new tab or window if he/she wants but you shouldn't be forcing him/her.
Validation enhances interoperability -- correct rendering in all user agents. However, web pages can, and do, fail validation and still display pretty much as expected in graphical Browsers anyway due to their built-in Markup discrepancy compensation -- in other words depending on the error compensating properties of individual Browsers. Validation does catch many easily corrected Markup errors and the resultant code is consequently easy to maintain or change. Pages containing invalid Markup may not display or function correctly in Screen Readers, BRAILLE interpreters and Textual Browsers or when incorporated into other applications. As we move toward the Semantic Web, the rigors of XML will result in a greater requirement for Valid Markup. Valid and well formed Markup also bespeaks careful craftsmanship and that appeals to many web authors. James
I await Teh Semantic Web. I like the idea, but I read a paper somewhere on Why It Will Fail, I think the page is called MetaCrap, and makes some good points-- the whole semantic web thing doesn't only depend on semantic, valid, well-written and well-formed pages, but that people bother to do everything else right too. I liked his example with the Plam Pilot. : )