In HTML documents, tags define the start and end of headings, paragraphs, lists, character highlighting and links. Most HTML elements are identified in a document as a start tag, which gives the element name and attributes, followed by the content, followed by the end tag. Start tags are delimited by < and >, and end tags are delimited by </ and >. For example:
<H1>This is a Heading</H1> <P>This is a paragraph.
Some elements appear as just a start tag. For example, to create a line break, you use <BR>. Additionally, the end tags of some other elements (e.g. P, LI, DT, DD) may be omitted.
The content of an element is a sequence of characters and nested elements. Some elements, such as anchors, cannot be nested. Anchors and character highlighting may be put inside other constructs.
The element name immediately follows the tag open delimiter. An element name consist of a letter followed by up to 72 letters, digits, periods, or hyphens. Names are not case sensitive. For example, H1 is equivalent to h1.
In a start tag, white space and attributes are allowed between the element name and the closing delimiter. An attribute typically consists of an attribute name, an equal sign, and a value (although some attributes may be just a value). White space is allowed around the equal sign.
The value of the attribute may be either:
In this example, a is the element name, href is the attribute name, and http://host/dir/file.html is the attribute value:
<A HREF="http://host/dir/file.html">
Some implementations consider any occurrence of the > character to signal the end of a tag. For compatibility with such implementations, when > appears in an attribute value, you may want to represent it with an entity or numeric character reference, such as:
<IMG SRC="eq1.ps" alt="a > b">
To put quotes inside of quotes, you use the character representation " as in:
<IMG SRC="image.ps" alt="First "real" example">
Alternatively, you can use single quotes if the outer quotes are double or vice versa, as in:
<IMG SRC="image.ps" alt="First 'real' example">
The length of an attribute value (after replacing entity and numeric character references) is limited to 1024 characters. This number is defined by the LITLEN parameter in the SGML declaration for HTML 3.0.
Attributes with a declared value of NAME (e.g. ISMAP, COMPACT) may be written using a minimized syntax. The markup:
<UL COMPACT="compact">
can be written as:
<UL COMPACT>
It is an accepted networking principle to be conservative in that which one produces, and liberal in that which one accepts. HTML parsers should be liberal except when verifying code. HTML generators should generate strictly conforming HTML.
The behavior of WWW applications reading HTML documents and discovering tag or attribute names which they do not understand should be to behave as though, in the case of a tag, the whole tag had not been there but its content had, or in the case of an attribute, that the attribute had not been present.
The characters between the tags represent text in the ISO-Latin-1 character set, which is a superset of ASCII. Because certain characters will be interpreted as markup, they should be represented by markup -- entity or numeric character references. See the Special Characters section of this specification for more information.
To include comments in an HTML document that will be ignored by the parser, surround them with <!-- and -->. After the comment delimiter, all text up to the next occurrence of --> is ignored. Hence comments cannot be nested. White space is allowed between the closing -- and >, but not between the opening <! and --.
For example:
<HEAD> <TITLE>HTML Guide: Recommended Usage</TITLE> <!-- Id: Text.html,v 1.6 1994/04/25 17:33:48 connolly Exp --> </HEAD>