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Content Notation
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The allowable content of the elements is shown using a content model syntax.
| Notation | Meaning |
|---|
| ( ) | ( and ) are grouping symbols around elements and/or other symbols
| | A | B | Either A or B occurs, but not both.
| | A , B | Both A and B occur, in that order.
| | A & B | Both A and B occur, in any order.
| | A? | A occurs zero or one time.
| | A* | A occurs zero or more times.
| | A+ | A occurs one or more times.
| | -A | A is prohibited at this point.
| | +A | A could appear multiple times at various positions.
| | EMPTY | this means that there must be nothing in the content;
elements with EMPTY content are called empty elements.
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Examples from HTML 2
The allowable content of the
HTML
element is:
( HEAD,
BODY )
This means that an HTML element must contain exactly one
HEAD followed by exactly
one BODY, in that order.
The allowable content of the
BODY element is:
( H1 | H2 | H3 | H4 | H5 | H6 | P | UL | OL | DIR | MENU | DL | PRE | BLOCKQUOTE | FORM | ISINDEX | HR | ADDRESS | IMG )*
This means that all those elements shown could be content of a BODY element. Don't misinterpret the
| symbol here--it does not mean that you can have to choose between having
a H1 or a H4 in the same BODY.
The notation means you can choose one of the elements within the (
and ). But the
* at the end means you can repeat this choice as often as you wish. So you can
have a H1, H2, or H5 in the same BODY.
The allowable content of the
HEAD element is:
( TITLE & ISINDEX? & BASE? ) +( META | LINK )
This means that a HEAD element can contain two groupings of possible elements. The first grouping consists of a TITLE element, an optional single ISINDEX, and an optional single BASE element. These can occur in any order.
In the second grouping, META
or LINK
could appear multiple times at various positions within HEAD.
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