An XML identifier (ID) |
This kind of attribute must have a value that is an XML NAME, which can consist of XML
name characters (alphabetical characters, digits, period, underscore, and hyphen), and cannot
start with a digit. Every ID attribute value in a single document must be unique
and provides a way to link or refer to its element (for example, using the <xref> element). ID attributes are generally
named @id.
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An identifier (IDREF) |
This kind of attribute must have a value that is the same as some ID value
in the same document. IDREFs appear on elements that refer to other elements
such as <xref>. IDREF attributes
are generally named @rid. Some @rid attributes are of type IDREFS, which is
simply a space-separated list of IDREF values.
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Text, numbers, or special characters (CDATA) |
These attributes can take any string value at all. If the attribute value is surrounded
by single quotes, then single quotes cannot appear inside; if the attribute value is
surrounded by double quotes, then double quotes cannot appear inside. In either case, the
prohibited character can instead be represented by an XML character reference such as
“'”. XML elements cannot be placed within attribute values.
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“xxx-type” attributes |
There are many attributes whose names end in “-type”. They
are generally CDATA attributes as described above. They are typically assigned
tokens as values, containing no spaces. Typically if there are spaces in the value, they
separate multiple independent tokens, all of which apply. For example, some element might be
both of type “important” and
“normative”, and be given type “important
normative”. In many cases, the Tag Library gives suggested values for such
attributes. Unless specifically stated otherwise, those values are not the only values
permitted.
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? |
means that the item is optional (zero or one)
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* |
means that the item may occur any number of times (zero or more)
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+ |
means that the item must occur at least once, but may occur any number of
times (one or more)
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a thick vertical bar on the left of the box |
means that the item is the “document element”: the top-most
element, such as <article> or <book> and is
therefore required.
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(no symbol) |
means that the item must occur exactly once
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˜ (a tilde) |
means that the item may have attribute
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a thick vertical bar on the right of the box |
means that the item is expanded elsewhere (For example, if an element is
permitted in multiple places within a certain parent element, there is little
point in repeating its information many times.
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