http://www.w3.org/html5,css - What does "<html xmlns="http://www.w3.org/1999/xhtml">" do?

本文探讨了HTML与XHTML之间的区别，特别是在文档声明、高度属性的表现及XML命名空间的应用方面。通过具体示例展示了不同标记语言的行为差异。

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You're mixing up HTML with XHTML.

Usually a declaration is used to distinguish between versions of HTMLish languages (in this case, HTML or XHTML).

Different markup languages will behave differently. My favorite example is height:100%. Look at the following in a browser:

XHTML

table { height:100%;background:yellow; }

How tall is this?

... and compare it to the following: (note the conspicuous lack of a declaration)

HTML (quirks mode)

table { height:100%;background:yellow; }

How tall is this?

You'll notice that the height of the table is drastically different, and the only difference between the 2 documents is the type of markup!

That's nice... now, what does do?

That doesn't answer your question though. Technically, the xmlns attribute is used by the root element of an XHTML document: (according to Wikipedia)

The root element of an XHTML document must be html, and must contain an xmlns attribute to associate it with the XHTML namespace.

You see, it's important to understand that XHTML isn't HTML but XML - a very different creature. (ok, a kind of different creature) The xmlns attribute is just one of those things the document needs to be valid XML. Why? Because someone working on the standard said so ;) (you can read more about XML namespaces on Wikipedia but I'm omitting that info 'cause it's not actually relevant to your question!)

But then why is fixing the CSS?

If structuring your document like so... (as you suggest in your comment)

[...]

... is fixing your document, it leads me to believe that you don't know that much about CSS and HTML (no offense!) and that the truth is that without it's behaving normally and with it's not - and you just think it is, because you're used to writing invalid HTML and thus working in quirks mode.

The above example I provided is an example of that same problem; most people think height:100% should result in the height of the