Is it acceptable for invalid XHTML?
I've noticed a lot of sites, SO included, use XHTML as their mark-up language and then fail to adhere to the spec. Just browsing the source for SO there are missing closing tags for paragraphs, invalid elements, etc.
So should tools (and developers) use the XHTML doctype if they are going to produce invalid mark up? And should browsers be more firm in their acceptance of poor mark-up?
And before anyone shouts hypocrite, my blog has one piece of invalid mark-up involving the captha (or it did the last time I checked) which involves styling the noscript tag.
There are many reasons to use valid markup. My favorite is that it allows you to use validation as a form of regression testing, preventing the markup equivalent of "delta rot" from leading to real rendering problems once the errors reach some critical mass. And really, it's just plain sloppy to allow "lazy" errors like typos and mis-nested/unclosed tags to accumulate. Valid markup is one way to identify passionate programmers.
There's also the issue of debugging: valid markup also gives you a stable baseline from which to work on the inevitable cross-browser compatibility woes. No web developer who values his time should begin debugging browser compatibility problems without first ensuring that the markup is at least syntactically valid—and any other invalid markup should have a good reason for being there.
All of that said, to answer your specific question, it's probably not worthwhile to use one of the XHTML doctypes unless you plan to produce valid (or at least well-formed) markup. XHTML's primary advantages are derived from the fact that XHTML is XML, allowing it to be processed and transformed by tools and technologies that work with XML. If you don't plan to make your XHTML well-formed XML, then there's little point in choosing that doctype. The latest HTML 4 spec will probably do everything you need, and it's much more forgiving.
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We should always try to make it validate according to standards. We'll be sure that the website will display and work fine on current browsers AND future browsers.
I don't think that, if you specify a doctype, there is any reason not to adhere to this doctype.
Using XHTML makes automated error detection easy, every change can be automatically checked for invalid markup. This prevents errors, especially when using automatically generated content. It is really easy for a web developer using a templating engine (JSP, ASP.NET StringTemplate, etcetera) to copy/paste one closing tag too little or too many. When this is your only error, it can be detected and fixed immediately. I once worked for a site that had 165 validation errors per page, of which 2 or 3 were actual bugs. These were hard to find in the clutter of other errors. Automatic validation would have prevented these errors at the source.
Needless to say, choosing a standard and sticking to it can never benefit interoperability with other systems (screen scrapers, screen readers, search engines) and I have never come across a situation where a valid semantic XHTML with CSS solution wasn't possible for all major browsers.
Obviously, when working with complex systems, it's not always possible to stick to your doctype, but this is mostly a result of improper communication between the different teams developing different parts of these systems, or, most likely, legacy systems. In the last case it's probably better to isolate these cases and change your doctype accordingly.
It's good to be pragmatic and not adhere to XHTML just because someone said so, regardless of costs, but with current knowledge about CSS and browsers, testing and validation tools, most of the time the benefits are much greater than the costs.
You can say that I have an OCD on XHTML validity. I find that most of the problems with the code not being valid comes from programmers not knowing the difference between HTML and XHTML. I've been writing 100% valid XHTML and CSS or a while now and have never had any major rendering problems with other browsers. If you keep everything valid, and don't try anything too exotic css wise, you will save yourself a ton of time in fixes.
I wouldn't use XHTML at all just to save myself the philosophical stress. It's not like any browsers are treating it like XHTML anyway.
Browsers will reject poor mark-up if the page is sent as application/xhtml+xml, but they rarely are. This is fine.