Why is XHTML 1.0 Transitional so popular?

Question

My company is looking to replace all websites in the group with a new CMS-based system and similar designs/styling, with E-Commerce functionality being added in a future phase. It's too big a job for me to do in a reasonable time-frame, so we are going to be inviting tenders from agencies.

I'm currently in the process of defining the technical requirements, and I'm intending to dictate that the selected system must have a Strict DOCTYPE and must trigger Standards mode (or Almost Standards Mode) in common browsers, or something to that effect [We have to allow Almost Standards Mode to cater for IE, obviously].

I've done a bit of homework an all of this - I don't want the spec to be limited by my ignorance, after all - but it won't surprise you all that I've found that 'current opinion' is completely divided on what is good practice.

There are plenty of people who advocate HTML4.01 Strict (fair enough), plenty of people who recommend XHTML1.0 Strict served as text/html (I'm OK with this too), some who recommend HTML5 but restricted to HTML4.01 tags (erm... still not sure if this is a good idea or not, but I see the principle), but also a not-inconsiderable number (including people on other SO threads) who recommend XHTML1.0 Transitional.

I just don't understand the reasoning for this... OK, you may happen to want to temporarily use something that been deprecated, and thus Transitional seems sensible, but some people recommend XHTML Transitional for new build.

After checking out other companies' sites for design inspiration, I've notice that many sites (if they have any DOCTYPE specified at all) will refer to a Transitional DTD. OK, we all know there is plenty of crap on the web, so perhaps I shouldn't draw too many conclusions. But checking out Web Design Agencies that we've come across, there are an amazing proportion of them (the vast majority of them, I would say) are using XHTML 1.0 Transitional.

Fine, so you don't necessarily have to be an expert to call yourself a Web Designer, but the sheer volume of Transitional layouts makes me wonder... Most of the sites seem to be otherwise reasonably designed (CSS layout, validating, accessible etc).

So, having finally got to the point(!), is there some reason why such a large proportion of these agencies are opting for Transitional DOCTYPEs? Am I missing something, something that I need to consider for my new sites?

Edit: Yeah, I realise the purpose of the Transitional DTD - I was just suspicious that so many otherwise-competent web developers are clinging on to deprecated markup. I wonder if you guys are correct and the answer is simply that they are a) are too lazy to get their own website to validate, or b) sticking with the default DTD of their preferred IDE.

The key re-assurance for me is that (according to your responses so far) I don't seem to be missing out on some key reason to use a Transitional DTD.

Edit 2: Regarding our CMS project - all the short-listed agencies thankfully seem to have their heads screwed on - Strict, valid and accessible.

Answer 1

I'm going to take a hard-nosed approach and say that Transitional doctypes are popular because some people are lazy or just don't care. Somebody told them they should validate their code, and transitional is the easiest to validate because it allows deprecated markup.

I agree with those who answered this previously and said that transitional doctypes are for transitioning to strict. Those are the only situations where I've used them.

Any new development should definitely be with a strict doctype. The choice of Strict vs. Transitional is more important than XHTML vs. HTML4. I would strongly recommend to you that in your technical requirements you require Strict.