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HTML5This section only describes the rules for resources labeled with an HTML MIME type. Rules for XML resources are discussed in the section below entitled "The XHTML syntax".
8.1 Writing HTML documentsThis section only applies to documents, authoring tools, and markup generators. In particular, it does not apply to conformance checkers; conformance checkers must use the requirements given in the next section ("parsing HTML documents").
Documents must consist of the following parts, in the given order:
html element.The various types of content mentioned above are described in the next few sections.
In addition, there are some restrictions on how character encoding declarations are to be serialized, as discussed in the section on that topic.
Space characters before the root html element, and space characters at the start of the html element and before the head element, will be dropped when the document is parsed; space characters after the root html element will be parsed as if they were at the end of the body element. Thus, space characters around the root element do not round-trip.
It is suggested that newlines be inserted after the DOCTYPE, after any comments that are before the root element, after the html element's start tag (if it is not omitted), and after any comments that are inside the html element but before the head element.
Many strings in the HTML syntax (e.g. the names of elements and their attributes) are case-insensitive, but only for characters in the ranges U+0041 to U+005A (LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and U+0061 to U+007A (LATIN SMALL LETTER A to LATIN SMALL LETTER Z). For convenience, in this section this is just referred to as "case-insensitive".
8.1.1 The DOCTYPEA DOCTYPE is a required preamble.
DOCTYPEs are required for legacy reasons. When omitted, browsers tend to use a different rendering mode that is incompatible with some specifications. Including the DOCTYPE in a document ensures that the browser makes a best-effort attempt at following the relevant specifications.
A DOCTYPE must consist of the following components, in this order:
<!DOCTYPE".html".In other words, <!DOCTYPE html>, case-insensitively.
For the purposes of HTML generators that cannot output HTML markup with the short DOCTYPE "<!DOCTYPE html>", a DOCTYPE legacy string may be inserted into the DOCTYPE (in the position defined above). This string must consist of:
SYSTEM".about:legacy-compat".In other words, <!DOCTYPE html SYSTEM "about:legacy-compat"> or <!DOCTYPE html SYSTEM 'about:legacy-compat'>, case-insensitively except for the part in single or double quotes.
The DOCTYPE legacy string should not be used unless the document is generated from a system that cannot output the shorter string.
To help authors transition from HTML4 and XHTML1, an obsolete permitted DOCTYPE string can be inserted into the DOCTYPE (in the position defined above). This string must consist of:
PUBLIC".-//W3C//DTD HTML 4.0//EN -//W3C//DTD HTML 4.0//EN http://www.w3.org/TR/REC-html40/strict.dtd -//W3C//DTD HTML 4.01//EN -//W3C//DTD HTML 4.01//EN http://www.w3.org/TR/html4/strict.dtd -//W3C//DTD XHTML 1.0 Strict//EN http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd -//W3C//DTD XHTML 1.1//EN http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd
A DOCTYPE containing an obsolete permitted DOCTYPE string is an obsolete permitted DOCTYPE. Authors should not use obsolete permitted DOCTYPEs, as they are unnecessarily long.
8.1.2 ElementsThere are five different kinds of elements: void elements, raw text elements, RCDATA elements, foreign elements, and normal elements.
area, base, br, col, command, embed, hr, img, input, keygen, link, meta, param, source, track, wbr
script, style
textarea, title
Tags are used to delimit the start and end of elements in the markup. Raw text, RCDATA, and normal elements have a start tag to indicate where they begin, and an end tag to indicate where they end. The start and end tags of certain normal elements can be omitted, as described later. Those that cannot be omitted must not be omitted. Void elements only have a start tag; end tags must not be specified for void elements. Foreign elements must either have a start tag and an end tag, or a start tag that is marked as self-closing, in which case they must not have an end tag.
The contents of the element must be placed between just after the start tag (which might be implied, in certain cases) and just before the end tag (which again, might be implied in certain cases). The exact allowed contents of each individual element depend on the content model of that element, as described earlier in this specification. Elements must not contain content that their content model disallows. In addition to the restrictions placed on the contents by those content models, however, the five types of elements have additional syntactic requirements.
Void elements can't have any contents (since there's no end tag, no content can be put between the start tag and the end tag).
Raw text elements can have text, though it has restrictions described below.
RCDATA elements can have text and character references, but the text must not contain an ambiguous ampersand. There are also further restrictions described below.
Foreign elements whose start tag is marked as self-closing can't have any contents (since, again, as there's no end tag, no content can be put between the start tag and the end tag). Foreign elements whose start tag is not marked as self-closing can have text, character references, CDATA sections, other elements, and comments, but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand.
The HTML syntax does not support namespace declarations, even in foreign elements.
For instance, consider the following HTML fragment:
<p> <svg> <metadata> <!-- this is invalid --> <cdr:license xmlns:cdr="http://www.example.com/cdr/metadata" name="MIT"/> </metadata> </svg> </p>
The innermost element, cdr:license, is actually in the SVG namespace, as the "xmlns:cdr" attribute has no effect (unlike in XML). In fact, as the comment in the fragment above says, the fragment is actually non-conforming. This is because the SVG specification does not define any elements called "cdr:license" in the SVG namespace.
Normal elements can have text, character references, other elements, and comments, but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand. Some normal elements also have yet more restrictions on what content they are allowed to hold, beyond the restrictions imposed by the content model and those described in this paragraph. Those restrictions are described below.
Tags contain a tag name, giving the element's name. HTML elements all have names that only use characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z, and U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z. 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.
8.1.2.1 Start tagsStart tags must have the following format:
End tags must have the following format:
Attributes for an element are expressed inside the element's start tag.
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 GREATER-THAN SIGN (>), U+002F SOLIDUS (/), and U+003D EQUALS SIGN (=) 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.
Attribute values are a mixture of text and character references, except with the additional restriction that the text cannot contain an ambiguous ampersand.
Attributes can be specified in four different ways:
Just the attribute name. The value is implicitly the empty string.
In the following example, the disabled attribute is given with the empty attribute syntax:
<input disabled>
If an attribute using the empty attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal space characters, any U+0022 QUOTATION MARK characters ("), U+0027 APOSTROPHE characters ('), U+003D EQUALS SIGN characters (=), U+003C LESS-THAN SIGN characters (<), U+003E GREATER-THAN SIGN characters (>), or U+0060 GRAVE ACCENT characters (`), and must not be the empty string.
In the following example, the value attribute is given with the unquoted attribute value syntax:
<input value=yes>
If an attribute using the unquoted attribute syntax is to be followed by another attribute or by the optional U+002F SOLIDUS character (/) allowed in step 6 of the start tag syntax above, then there must be a space character separating the two.
The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by a single U+0027 APOSTROPHE character ('), followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal U+0027 APOSTROPHE characters ('), and finally followed by a second single U+0027 APOSTROPHE character (').
In the following example, the type attribute is given with the single-quoted attribute value syntax:
<input type='checkbox'>
If an attribute using the single-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by a single U+0022 QUOTATION MARK character ("), followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal U+0022 QUOTATION MARK characters ("), and finally followed by a second single U+0022 QUOTATION MARK character (").
In the following example, the name attribute is given with the double-quoted attribute value syntax:
<input name="be evil">
If an attribute using the double-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.
There must never be two or more attributes on the same start tag whose names are an ASCII case-insensitive match for each other.
When a foreign element has one of the namespaced attributes given by the local name and namespace of the first and second cells of a row from the following table, it must be written using the name given by the third cell from the same row.
No other namespaced attribute can be expressed in the HTML syntax.
8.1.2.4 Optional tagsCertain tags can be omitted.
Omitting an element's start tag does not mean the element is not present; it is implied, but it is still there. An HTML document always has a root html element, even if the string <html> doesn't appear anywhere in the markup.
An html element's start tag may be omitted if the first thing inside the html element is not a comment.
An html element's end tag may be omitted if the html element is not immediately followed by a comment.
A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.
A head element's end tag may be omitted if the head element is not immediately followed by a space character or a comment.
A body element's start tag may be omitted if the element is empty, or if the first thing inside the body element is not a space character or a comment, except if the first thing inside the body element is a script or style element.
A body element's end tag may be omitted if the body element is not immediately followed by a comment.
A li element's end tag may be omitted if the li element is immediately followed by another li element or if there is no more content in the parent element.
A dt element's end tag may be omitted if the dt element is immediately followed by another dt element or a dd element.
A dd element's end tag may be omitted if the dd element is immediately followed by another dd element or a dt element, or if there is no more content in the parent element.
A p element's end tag may be omitted if the p element is immediately followed by an address, article, aside, blockquote, dir, div, dl, fieldset, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, menu, nav, ol, p, pre, section, table, or ul, element, or if there is no more content in the parent element and the parent element is not an a element.
An rt element's end tag may be omitted if the rt element is immediately followed by an rt or rp element, or if there is no more content in the parent element.
An rp element's end tag may be omitted if the rp element is immediately followed by an rt or rp element, or if there is no more content in the parent element.
An optgroup element's end tag may be omitted if the optgroup element is immediately followed by another optgroup element, or if there is no more content in the parent element.
An option element's end tag may be omitted if the option element is immediately followed by another option element, or if it is immediately followed by an optgroup element, or if there is no more content in the parent element.
A colgroup element's start tag may be omitted if the first thing inside the colgroup element is a col element, and if the element is not immediately preceded by another colgroup element whose end tag has been omitted. (It can't be omitted if the element is empty.)
A colgroup element's end tag may be omitted if the colgroup element is not immediately followed by a space character or a comment.
A thead element's end tag may be omitted if the thead element is immediately followed by a tbody or tfoot element.
A tbody element's start tag may be omitted if the first thing inside the tbody element is a tr element, and if the element is not immediately preceded by a tbody, thead, or tfoot element whose end tag has been omitted. (It can't be omitted if the element is empty.)
A tbody element's end tag may be omitted if the tbody element is immediately followed by a tbody or tfoot element, or if there is no more content in the parent element.
A tfoot element's end tag may be omitted if the tfoot element is immediately followed by a tbody element, or if there is no more content in the parent element.
A tr element's end tag may be omitted if the tr element is immediately followed by another tr element, or if there is no more content in the parent element.
A td element's end tag may be omitted if the td element is immediately followed by a td or th element, or if there is no more content in the parent element.
A th element's end tag may be omitted if the th element is immediately followed by a td or th element, or if there is no more content in the parent element.
However, a start tag must never be omitted if it has any attributes.
8.1.2.5 Restrictions on content modelsFor historical reasons, certain elements have extra restrictions beyond even the restrictions given by their content model.
A table element must not contain tr elements, even though these elements are technically allowed inside table elements according to the content models described in this specification. (If a tr element is put inside a table in the markup, it will in fact imply a tbody start tag before it.)
A single newline may be placed immediately after the start tag of pre and textarea elements. This does not affect the processing of the element. The otherwise optional newline must be included if the element's contents themselves start with a newline (because otherwise the leading newline in the contents would be treated like the optional newline, and ignored).
The following two pre blocks are equivalent:
<pre>Hello</pre>
<pre>8.1.2.6 Restrictions on the contents of raw text and RCDATA elements
Hello</pre>
The text in raw text and RCDATA elements must not contain any occurrences of the string "</" (U+003C LESS-THAN SIGN, U+002F SOLIDUS) followed by characters that case-insensitively match the tag name of the element followed by one of U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), U+0020 SPACE, U+003E GREATER-THAN SIGN (>), or U+002F SOLIDUS (/).
Text is allowed inside elements, attribute values, and comments. Text must consist of Unicode characters. Text must not contain U+0000 characters. Text must not contain permanently undefined Unicode characters (noncharacters). Text must not contain control characters other than space characters. Extra constraints are placed on what is and what is not allowed in text based on where the text is to be put, as described in the other sections.
8.1.3.1 NewlinesNewlines in HTML may be represented either as U+000D CARRIAGE RETURN (CR) characters, U+000A LINE FEED (LF) characters, or pairs of U+000D CARRIAGE RETURN (CR), U+000A LINE FEED (LF) characters in that order.
Where character references are allowed, a character reference of a U+000A LINE FEED (LF) character (but not a U+000D CARRIAGE RETURN (CR) character) also represents a newline.
8.1.4 Character referencesIn certain cases described in other sections, text may be mixed with character references. These can be used to escape characters that couldn't otherwise legally be included in text.
Character references must start with a U+0026 AMPERSAND character (&). Following this, there are three possible kinds of character references:
The numeric character reference forms described above are allowed to reference any Unicode code point other than U+0000, U+000D, permanently undefined Unicode characters (noncharacters), and control characters other than space characters.
An ambiguous ampersand is a U+0026 AMPERSAND character (&) that is followed by one or more characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z, and U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z, followed by a U+003B SEMICOLON character (;), where these characters do not match any of the names given in the named character references section.
8.1.5 CDATA sectionsCDATA sections must consist of the following components, in this order:
<![CDATA[".]]>".]]>".CDATA sections can only be used in foreign content (MathML or SVG). In this example, a CDATA section is used to escape the contents of an ms element:
<p>You can add a string to a number, but this stringifies the number:</p> <math> <ms><![CDATA[x<y]]></ms> <mo>+</mo> <mn>3</mn> <mo>=</mo> <ms><![CDATA[x<y3]]></ms> </math>
must start with the four character sequence U+003C LESS-THAN SIGN, U+0021 EXCLAMATION MARK, U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS (<!--). Following this sequence, the comment may have text, with the additional restriction that the text must not start with a single U+003E GREATER-THAN SIGN character (>), nor start with a U+002D HYPHEN-MINUS character (-) followed by a U+003E GREATER-THAN SIGN (>) character, nor contain two consecutive U+002D HYPHEN-MINUS characters (--), nor end with a U+002D HYPHEN-MINUS character (-). Finally, the comment must be ended by the three character sequence U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS, U+003E GREATER-THAN SIGN (-->).
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