Namespace and HTML 5 - html

Question

In the HTML specs one can find the following line:
In the HTML syntax, namespace prefixes and namespace declarations do not have the same effect as in XML. For instance, the colon has no special meaning in HTML element names.
After looking into the Grammar definition there are the following sections:
On tag names it states:
Tags contain a tag name, giving the element's name. HTML elements all have names that only use alphanumeric ASCII characters. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.
This leaves almost no room for interpretation. There is no underscore or dollar sign here. Also there is no ':' making it impossible to legally express names spaces. It also makes it possible to use only a number like <1> but then the grammar states:
Uppercase ASCII letter
Create a new start tag token, set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point), then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter
Create a new start tag token, set its tag name to the current input character, then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
So we are only left to something like <a1234>.
On attribute names it states:
Attributes have a name and a value. Attribute names must consist of one or more characters other than the space characters, U+0000 NULL, U+0022 QUOTATION MARK ("), U+0027 APOSTROPHE ('), ">" (U+003E), "/" (U+002F), and "=" (U+003D) characters, the control characters, and any characters that are not defined by Unicode. In the HTML syntax, attribute names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that are an ASCII case-insensitive match for the attribute's name.
Reading this it seems this is possible:
<div ::::::="hello" $_$="dollar"></div>
From all this using namespaces for tag names is forbidden and for attributes it's mere a convention you may follow but do not need to.
So to put it simple namespace for HTML 5 does not exist and at least for the tag name can not be emulated and we have no underscore and no dot or something alike.
Is this correct? On the other hand HTML 5 specs state that we are free to add xmlns attributes to the elements making it possible to clearly introduce new namespaces. How does this fit?
[Update]
I rechecked the specification using the single page version of the specs and it actually stats that the name space declartion is allowed for xhtml left overs but it actually has to be ignored so no name spaces for us. Sad thing.
[/Update]
So the only question left is, if there is no ':' or anything else what can I legally do with element tag names. Can I use some special one I have made up. Remember we habe a relaxed specification for the parser here. The parser should be build in a way that it can handle unkown element tags. The question here is, how do they handle unknown element tags?

Answer 1

The HTML 5 specification allows only xmlns name space attributes with regard to the xhtml document specification. Those name spaces are ignored and not valued.
The tag name section of the specs is a bit confusing since it only talks about HTML elements. The parser section for tag names reads:
8.2.4.10 Tag name state
Consume the next input character:
"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE
-> Switch to the before attribute name state.
"/" (U+002F)
-> Switch to the self-closing start tag state.
">" (U+003E)
-> Switch to the data state. Emit the current tag token.
Uppercase ASCII letter
-> Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name.
U+0000 NULL
-> Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current tag token's tag name.
EOF
-> Parse error. Switch to the data state. Reconsume the EOF character.
Anything else
-> Append the current input character to the current tag token's tag name.
The last line is the important part. Also the specification only states for HTML elements defined as those. Therefore we are free to do things like and it is considered a valid element but not a valid HTML element. The question is how a browser or Editor reacts toward this character soup. But again it is a valid element name but not a valid HTML element name.