The main problem is that DocType is not ready to handle the SO HTML/CSS traffic. They should have gone into beta with some SO users until they got the kinks worked out.
As it is currently, they are using some bizarre sort algorithm for questions and you cannot sort by last updated/most active. The only way to find new answers is to page through the site looking at timestamps.
Also if a question has a 'best answer' selected there is no indication when it gets a new answer. In order to know a 'best answer' question has new answers is to keep track of the number of answers it had before.
If you to push all HTML/CSS questions to DocType, people are going to try it and end up leaving the 'SO family' to answer questions forums that aren't so hard to use.
edit to respond to Matthew's comment: 'I'd argue that Doctype is very easy to use, it's just different to StackOverflow (that you're used to).'
I think DocType is easy to use to ask a question. I love the repositories of screenshots and code. But to attract and keep people who will answer questions, it has to be even easier for them.
When I go to a forum, I want to be able to scan down a list of new or updated questions/topics. I don't want to have to hunt down new questions or interesting questions that may have new answers. I don't want to have to page down four times to see ten questions. I don't want to have to resize my font every time I open DocType in a new tab. I don't want to have to scroll up because I cannot click on a question that is covered by the black footer but it goes off the screen when I page down.
I think DocType has potential and will eventually be a really cool site. I just don't think it is ready to host all of the SO HTML/CSS questions.