A simple XHTML document, with the minimum of required tags: and xmlns attribute has a http link.
If I am working on localhost and if I don't have an Internet connection, can't I work on XHTML?
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Title of the document</title>
</head>
<body>
The content of the document......
</body>
</html>
The XML namespace is just there to identify elements based on a namespace, which happens to be in a URI format. See the XML Namespaces spec. In this case, http://www.w3.org/1999/xhtml is the namespace for XHTML; an XML processor will be able to use this information to identify which elements are in the XHTML namespace (identified by that URL).
The browser isn't going to attempt to request the URL that's listed in the xmlns attribute. Even if it does, it won't receive anything useful anyway; visit http://www.w3.org/1999/xhtml in your browser and see for yourself.
So there's nothing to worry about, even if you're working on a development machine with no Internet connection. You'll still be able to write valid XHTML.
来源：https://stackoverflow.com/questions/13539253/without-internet-connection-how-xmlns-attribute-can-work-and-understand-by-brow