6 Basic HTML data types
This section of the specification describes the basic data types that may
appear as an element's content or an attribute's value.
For introductory information about reading the HTML DTD, please consult the
SGML tutorial.
Each attribute definition includes information about the
case-sensitivity of its values. The case information is presented
with the following keys:
- CS
- The value is case-sensitive (i.e., user agents interpret "a" and "A"
differently).
- CI
- The value is case-insensitive (i.e., user agents interpret "a" and "A" as
the same).
- CN
- The value is not subject to case changes, e.g., because it is a number or a
character from the document character set.
- CA
- The element or attribute definition itself gives case information.
- CT
- Consult the type definition for details about case-sensitivity.
If an attribute value is a list, the keys apply to every value in the list,
unless otherwise indicated.
6.2 SGML basic types
The document type definition specifies the
syntax of HTML element content and attribute values using SGML tokens (e.g.,
PCDATA, CDATA, NAME, ID, etc.). See [ISO8879] for their full
definitions. The following is a summary of key information:
- CDATA is a sequence of characters from
the document character set and may include character entities. User agents
should interpret attribute values as follows:
- Replace character entities with characters,
- Ignore line feeds,
- Replace each carriage return or tab with a single space.
User agents may ignore leading and trailing white space in CDATA attribute
values (e.g., " myval " may be interpreted as
"myval"). Authors should not declare attribute values with leading or trailing
white space.
For some HTML 4 attributes with CDATA attribute values, the specification
imposes further constraints on the set of legal values for the attribute that
may not be expressed by the DTD.
Although the STYLE and SCRIPT elements use CDATA for
their data model, for these elements, CDATA must be handled differently by user agents.
Markup and entities must be treated as raw text and passed to the application
as is. The first occurrence of the character sequence "</" (end-tag open
delimiter) is treated as terminating the end of the element's content. In valid
documents, this would be the end tag for the element.
-
ID and NAME tokens must
begin with a letter ([A-Za-z]) and may be followed by any number of letters,
digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods
(".").
- IDREF and
IDREFS are references to ID tokens defined by other
attributes. IDREF is a single token and IDREFS is a space-separated list of
tokens.
- NUMBER tokens must contain at least
one digit ([0-9]).
A number of attributes ( %Text; in the DTD) take text that is meant to
be "human readable". For introductory information about attributes, please
consult the tutorial discussion of
attributes.
This specification uses the term URI as defined in [URI] (see also
[RFC1630]).
Note that URIs include URLs (as defined in [RFC1738] and
[RFC1808]).
Relative URIs are resolved to full URIs using a base URI.
[RFC1808], section 3, defines the normative algorithm for this process. For
more information about base URIs, please consult the section on base URIs in the chapter on links.
URIs are represented in the DTD by the parameter entity %URI;.
URIs in general are case-sensitive.
There may be URIs, or parts of URIs, where case doesn't matter (e.g., machine
names), but identifying these may not be easy. Users should always consider
that URIs are case-sensitive (to be on the safe side).
Please consult the appendix for information about non-ASCII characters in URI attribute
values.
The attribute value type "color" (%Color;) refers to color definitions as
specified in [SRGB]. A color value may either be a hexadecimal number
(prefixed by a hash mark) or one of the following sixteen color names. The
color names are case-insensitive.
Color names and sRGB values
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Black = "#000000" |
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Green = "#008000"
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Silver = "#C0C0C0" |
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Lime = "#00FF00"
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Gray = "#808080" |
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Olive = "#808000"
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White = "#FFFFFF" |
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Yellow = "#FFFF00"
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Maroon = "#800000" |
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Navy = "#000080"
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Red = "#FF0000" |
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Blue = "#0000FF"
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Purple = "#800080" |
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Teal = "#008080"
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Fuchsia = "#FF00FF" |
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Aqua = "#00FFFF"
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Thus, the color values "#800080" and "Purple" both refer to the color
purple.
6.5.1 Notes on using colors
Although colors can add significant amounts of information to documents and
make them more readable, please consider the following guidelines when
including color in your documents:
- The use of HTML elements and attributes for specifying color is deprecated. You are encouraged to use style sheets instead.
- Don't use color combinations that cause problems for people with color
blindness in its various forms.
- If you use a background image or set the background color, then be sure to
set the various text colors as well.
- Colors specified with the BODY and
FONT elements and bgcolor on tables look different
on different platforms (e.g., workstations, Macs, Windows, and LCD panels vs.
CRTs), so you shouldn't rely entirely on a specific effect. In the future,
support for the [SRGB] color model together with ICC color profiles should
mitigate this problem.
- When practical, adopt common conventions to minimize user confusion.
6.6 Lengths
HTML specifies three types of length values for attributes:
- Pixels: The value (%Pixels; in
the DTD) is an integer that represents the number of pixels of the canvas
(screen, paper). Thus, the value "50" means fifty pixels. For normative
information about the definition of a pixel, please consult [CSS1].
- Length: The value (%Length; in
the DTD) may be either a %Pixel; or a percentage of the available horizontal or
vertical space. Thus, the value "50%" means half of the available space.
- MultiLength: The value (
%MultiLength; in the DTD) may be a %Length; or a
relative length. A relative length has the form "i*", where
"i" is an integer. When allotting space among elements competing for that
space, user agents allot pixel and percentage lengths first, then divide up
remaining available space among relative lengths. Each relative length receives
a portion of the available space that is proportional to the integer preceding
the "*". The value "*" is equivalent to "1*". Thus, if 60 pixels of space are
available after the user agent allots pixel and percentage space, and the
competing relative lengths are 1*, 2*, and 3*, the 1* will be alloted 10
pixels, the 2* will be alloted 20 pixels, and the 3* will be alloted 30
pixels.
Length values are case-neutral.
Note. A "media type" (defined in [RFC2045] and [RFC2046])
specifies the nature of a linked resource. This specification employs the term
"content type" rather than "media type" in accordance with current usage.
Furthermore, in this specification, "media type" may refer to the media where a user agent renders a
document.
This type is represented in the DTD by
%ContentType;.
Content types are case-insensitive.
Examples of content types include "text/html", "image/png", "image/gif",
"video/mpeg", "text/css", and "audio/basic". For the current list of registered
MIME types, please consult [MIMETYPES].
The value of attributes whose type is a language code (
%LanguageCode in the DTD) refers to a language code as specified by
[RFC1766], section 2. For information on specifying language codes in HTML,
please consult the section on language
codes. Whitespace is not allowed within the language-code.
Language codes are
case-insensitive.
The "charset" attributes (%Charset in the DTD) refer to a character encoding as described in the
section on character encodings. Values
must be strings (e.g., "euc-jp") from the IANA registry (see
[CHARSETS] for a complete list).
Names of character encodings are
case-insensitive.
User agents must follow the steps set out in the section on specifying character encodings in order
to determine the character encoding of an external resource.
Certain attributes call for a single character from the document character set. These attributes take
the
%Character type in the DTD.
Single characters may be specified with
character references (e.g., "&").
[ISO8601] allows many options and variations in the representation of dates
and times. The current specification uses one of the formats described in the
profile [DATETIME] for its definition of legal date/time strings ( %Datetime
in the DTD).
The format is:
YYYY-MM-DDThh:mm:ssTZD
where:
YYYY = four-digit year
MM = two-digit month (01=January, etc.)
DD = two-digit day of month (01 through 31)
hh = two digits of hour (00 through 23) (am/pm NOT allowed)
mm = two digits of minute (00 through 59)
ss = two digits of second (00 through 59)
TZD = time zone designator
The time zone designator is one of:
- Z
- indicates UTC (Coordinated Universal Time). The "Z" must be uppercase.
- +hh:mm
- indicates that the time is a local time which is hh hours and
mm minutes ahead of UTC.
- -hh:mm
- indicates that the time is a local time which is hh hours and
mm minutes behind UTC.
Exactly the components shown here must be present, with exactly this
punctuation. Note that the "T" appears literally in the string (it
must be uppercase), to indicate the beginning of the time element, as specified
in
[ISO8601]
If a generating application does not know the time to the second, it may use
the value "00" for the seconds (and minutes and hours if necessary).
Note. [DATETIME] does not
address the issue of leap seconds.
Authors may use the following recognized link
types, listed here with their conventional interpretations. In the
DTD, %LinkTypes refers to a
space-separated list of link types. White space characters are not permitted
within link types.
These link types are
case-insensitive, i.e., "Alternate" has the same meaning as
"alternate".
User agents, search engines, etc. may interpret these link types in a
variety of ways. For example, user agents may provide access to linked
documents through a navigation bar.
- Alternate
- Designates substitute versions for the document in which the link occurs.
When used together with the lang attribute, it implies a translated
version of the document. When used together with the
media attribute, it implies a version designed for a different
medium (or media).
- Stylesheet
- Refers to an external style sheet. See the section on external style sheets for details.
This is used together with the link type "Alternate" for user-selectable
alternate style sheets.
- Start
- Refers to the first document in a collection of documents. This link type
tells search engines which document is considered by the author to be the
starting point of the collection.
- Next
- Refers to the next document in a linear sequence of documents. User agents
may choose to preload the "next" document, to reduce the perceived load
time.
- Prev
- Refers to the previous document in an ordered series of documents. Some
user agents also support the synonym "Previous".
- Contents
- Refers to a document serving as a table of contents. Some user agents also
support the synonym ToC (from "Table of Contents").
- Index
- Refers to a document providing an index for the current document.
- Glossary
- Refers to a document providing a glossary of terms that pertain to the
current document.
- Copyright
- Refers to a copyright statement for the current document.
- Chapter
- Refers to a document serving as a chapter in a collection of
documents.
- Section
- Refers to a document serving as a section in a collection of
documents.
- Subsection
- Refers to a document serving as a subsection in a collection of
documents.
- Appendix
- Refers to a document serving as an appendix in a collection of
documents.
- Help
- Refers to a document offering help (more information, links to other
sources information, etc.)
- Bookmark
- Refers to a bookmark. A bookmark is a link to a key entry point within an
extended document. The
title attribute may be used, for example, to
label the bookmark. Note that several bookmarks may be defined in each
document.
Authors may wish to define additional link
types not described in this specification. If they do so, they
should use a profile to cite the
conventions used to define the link types. Please see the
profile attribute of the HEAD element for more
details.
For further discussions about link types, please consult the section on links in HTML documents.
The following is a list of recognized media
descriptors ( %MediaDesc in the DTD).
- screen
- Intended for non-paged computer screens.
- tty
- Intended for media using a fixed-pitch character grid, such as teletypes,
terminals, or portable devices with limited display capabilities.
- tv
- Intended for television-type devices (low resolution, color, limited
scrollability).
- projection
- Intended for projectors.
- handheld
- Intended for handheld devices (small screen, monochrome, bitmapped
graphics, limited bandwidth).
- print
- Intended for paged, opaque material and for documents viewed on screen in
print preview mode.
- braille
- Intended for braille tactile feedback devices.
- aural
- Intended for speech synthesizers.
- all
- Suitable for all devices.
Future versions of HTML may introduce new values and may allow parameterized
values. To facilitate the introduction of these extensions, conforming user
agents must be able to parse the
media attribute value as follows:
- The value is a comma-separated list of entries. For example,
media="screen, 3d-glasses, print and resolution > 90dpi"
is mapped to:
"screen"
"3d-glasses"
"print and resolution > 90dpi"
- Each entry is truncated just before the first character that isn't a US
ASCII letter [a-zA-Z] (ISO 10646 hex 41-5a, 61-7a), digit [0-9] (hex 30-39), or
hyphen (hex 2d). In the example, this gives:
"screen"
"3d-glasses"
"print"
- A
case-sensitive match is then made with the set of media types
defined above. User agents may ignore entries that don't match. In the example
we are left with screen and print.
Note. Style sheets may include media-dependent
variations within them (e.g., the CSS @media construct). In
such cases it may be appropriate to use "media=all".
Script data ( %Script; in the
DTD) can be the content of the SCRIPT element and the value of
intrinsic event attributes. User
agents must not evaluate script data as HTML markup but instead must pass it on
as data to a script engine.
The case-sensitivity of script data depends on the
scripting language.
Please note that script data that is element content may not contain character references, but script data that is
the value of an attribute may contain them. The appendix provides further
information about
specifying non-HTML data.
Style sheet data (%StyleSheet; in the
DTD) can be the content of the
STYLE element and the value of the
style attribute. User agents must not evaluate style data as HTML
markup.
The case-sensitivity of style data depends on the style
sheet language.
Please note that style sheet data that is element content may not contain character references, but style sheet data
that is the value of an attribute may contain them. The appendix provides
further information about
specifying non-HTML data.
Except for the reserved names listed below, frame target names (%FrameTarget; in the DTD) must
begin with an alphabetic character (a-zA-Z). User agents should ignore all
other target names.
The following target
names are reserved and have special meanings.
- _blank
- The user agent should load the designated document in a new, unnamed
window.
- _self
- The user agent should load the document in the same frame as the element
that refers to this target.
- _parent
- The user agent should load the document into the immediate
FRAMESET parent of the current frame. This value is equivalent to
_self if the current frame has no parent.
- _top
- The user agent should load the document into the full, original window
(thus canceling all other frames). This value is equivalent to _self
if the current frame has no parent.
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