17 The HTML syntax

This 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".

17.1 Writing HTML documents

Documents must consist of the following parts, in the given order:

  1. Optionally, a single U+FEFF BYTE ORDER MARK (BOM) character.
  2. Any number of comments and space characters.
  3. A DOCTYPE.
  4. Any number of comments and space characters.
  5. The root element, in the form of an html element.
  6. Any number of comments and space characters.

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 uppercase ASCII letters and lowercase ASCII letters. For convenience, in this section this is just referred to as "case-insensitive".

17.1.1 The DOCTYPE

A 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:

  1. A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".
  2. One or more space characters.
  3. A string that is an ASCII case-insensitive match for the string "html".
  4. Optionally, a DOCTYPE legacy string or an obsolete permitted DOCTYPE string (defined below).
  5. Zero or more space characters.
  6. A U+003E GREATER-THAN SIGN character (>).

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:

  1. One or more space characters.
  2. A string that is an ASCII case-insensitive match for the string "SYSTEM".
  3. One or more space characters.
  4. A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the quote mark).
  5. The literal string "about:legacy-compat".
  6. A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled quote mark).

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:

  1. One or more space characters.
  2. A string that is an ASCII case-insensitive match for the string "PUBLIC".
  3. One or more space characters.
  4. A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the first quote mark).
  5. The string from one of the cells in the first column of the table below. The row to which this cell belongs is the selected row.
  6. A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled first quote mark).
  7. If the cell in the second column of the selected row is not blank, one or more space characters.
  8. If the cell in the second column of the selected row is not blank, a U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the third quote mark).
  9. If the cell in the second column of the selected row is not blank, the string from the cell in the second column of the selected row.
  10. If the cell in the second column of the selected row is not blank, a matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled third quote mark).
Allowed values for public and system identifiers in an obsolete permitted DOCTYPE string.
Public identifier System identifier
-//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.

17.1.2 Elements

There are five different kinds of elements: void elements, raw text elements, escapable raw text elements, foreign elements, and normal elements.

Void elements
area, base, br, col, embed, hr, img, input, keygen, link, menuitem, meta, param, source, track, wbr
Raw text elements
script, style
escapable raw text elements
textarea, title
Foreign elements
Elements from the MathML namespace and the SVG namespace.
Normal elements
All other allowed HTML elements are normal elements.

Tags are used to delimit the start and end of elements in the markup. Raw text, escapable raw text, 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 below in the section on optional tags. 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.

Escapable raw text 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 alphanumeric ASCII characters. 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.

17.1.2.1 Start tags

Start tags must have the following format:

  1. The first character of a start tag must be a U+003C LESS-THAN SIGN character (<).
  2. The next few characters of a start tag must be the element's tag name.
  3. If there are to be any attributes in the next step, there must first be one or more space characters.
  4. Then, the start tag may have a number of attributes, the syntax for which is described below. Attributes must be separated from each other by one or more space characters.
  5. After the attributes, or after the tag name if there are no attributes, there may be one or more space characters. (Some attributes are required to be followed by a space. See the attributes section below.)
  6. Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single U+002F SOLIDUS character (/). This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
  7. Finally, start tags must be closed by a U+003E GREATER-THAN SIGN character (>).
17.1.2.2 End tags

End tags must have the following format:

  1. The first character of an end tag must be a U+003C LESS-THAN SIGN character (<).
  2. The second character of an end tag must be a U+002F SOLIDUS character (/).
  3. The next few characters of an end tag must be the element's tag name.
  4. After the tag name, there may be one or more space characters.
  5. Finally, end tags must be closed by a U+003E GREATER-THAN SIGN character (>).
17.1.2.3 Attributes

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:

Empty attribute syntax

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.

Unquoted attribute value syntax

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.

Single-quoted attribute value syntax

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.

Double-quoted attribute value syntax

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.

Local name Namespace Attribute name
actuate XLink namespace xlink:actuate
arcrole XLink namespace xlink:arcrole
href XLink namespace xlink:href
role XLink namespace xlink:role
show XLink namespace xlink:show
title XLink namespace xlink:title
type XLink namespace xlink:type
base XML namespace xml:base
lang XML namespace xml:lang
space XML namespace xml:space
xmlns XMLNS namespace xmlns
xlink XMLNS namespace xmlns:xlink

No other namespaced attribute can be expressed in the HTML syntax.

Whether the attributes in the table above are conforming or not is defined by other specifications (e.g. the SVG and MathML specifications); this section only describes the syntax rules if the attributes are serialized using the HTML syntax.

17.1.2.4 Optional tags

Certain tags can be omitted.

Omitting an element's start tag in the situations described below does not mean the element is not present; it is implied, but it is still there. For example, 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 meta, link, script, style, or template element.

A body element's end tag may be omitted if the body element is not immediately followed by a comment.

An 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, div, dl, fieldset, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, main, 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.

17.1.2.5 Restrictions on content models

For 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>
Hello</pre>
17.1.2.6 Restrictions on the contents of raw text and escapable raw text elements

The text in raw text and escapable raw text 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 (/).

17.1.3 Text

Text is allowed inside elements, attribute values, and comments. 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.

17.1.3.1 Newlines

Newlines 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.

17.1.4 Character references

In 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:

Named character references
The ampersand must be followed by one of the names given in the named character references section, using the same case.
Decimal numeric character reference
The ampersand must be followed by a U+0023 NUMBER SIGN character (#), followed by one or more ASCII digits, representing a base-ten integer that corresponds to a Unicode code point that is allowed according to the definition below. The digits must then be followed by a U+003B SEMICOLON character (;).
Hexadecimal numeric character reference
The ampersand must be followed by a U+0023 NUMBER SIGN character (#), which must be followed by either a U+0078 LATIN SMALL LETTER X character (x) or a U+0058 LATIN CAPITAL LETTER X character (X), which must then be followed by one or more ASCII hex digits, representing a base-sixteen integer that corresponds to a Unicode code point that is allowed according to the definition below. The digits must then be followed by a U+003B SEMICOLON character (;).

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), surrogates (U+D800–U+DFFF), and control characters other than space characters.

An ambiguous ampersand is a U+0026 AMPERSAND character (&) that is followed by one or more alphanumeric ASCII characters, followed by a U+003B SEMICOLON character (;), where these characters do not match any of the names given in the named character references section.

17.1.5 CDATA sections

CDATA sections must consist of the following components, in this order:

  1. The string "<![CDATA[".
  2. Optionally, text, with the additional restriction that the text must not contain the string "]]>".
  3. The string "]]>".

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>

17.1.6 Comments

Comments 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 (-->).