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Appendix A. HTML Grammar

For the most part, the exact syntax of an HTML or XHTML document is not rigidly enforced by a browser. This gives authors wide latitude in creating documents and gives rise to documents that work on most browsers, but are actually incompatible with the HTML and XHTML standards. Stick to the standards unless your documents are fly-by-night affairs.

The standards explicitly define the ordering and nesting of tags and document elements. This syntax is embedded within the appropriate Document Type Definition and is not readily understood by those not versed in SGML (for HTML 4.01, see Appendix D, "The HTML 4.01 DTD") or XML (for XHTML 1.0, see Appendix E, "The XHTML 1.0 DTD"). Accordingly, we provide an alternate definition of the allowable HTML and XHTML syntax, using a fairly common tool called a "grammar."

Grammar, whether it defines English sentences or HTML documents, is just a set of rules that indicates the order of language elements. These language elements can be divided into two sets: terminal (the actual words of the language) and nonterminal (all other grammatical rules). In HTML and XHTML, the words correspond to the embedded markup tags and text in a document.

To use the grammar to create a valid document, follow the order of the rules to see where the tags and text may be placed to create a valid document.

A.1. Grammatical Conventions

We use a number of typographic and punctuation conventions to make our grammar easy to understand.

A.1.2. Punctuation Conventions

Each rule in the grammar starts with the rule's name, followed by the replacement symbol (::=) and the rule's value. We've intentionally kept the grammar simple, but we do use three punctuation elements to denote alternation, repetition, and optional elements in the grammar.

A.1.3. More Details

Our grammar stops at the tag level; it does not delve further to show the syntax of each tag, including tag attributes. For these details, refer to the Quick Reference card included with this book.

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