Character encodings in HTML

Last updated

While Hypertext Markup Language (HTML) has been in use since 1991, HTML 4.0 from December 1997 was the first standardized version where international characters were given reasonably complete treatment. When an HTML document includes special characters outside the range of seven-bit ASCII, two goals are worth considering: the information's integrity, and universal browser display.

Contents

Specifying the document's character encoding

There are two general ways to specify which character encoding is used in the document.

First, the web server can include the character encoding or "charset" in the Hypertext Transfer Protocol (HTTP) Content-Type header, which would typically look like this: [1] 

Content-Type: text/html; charset=utf-8

This method gives the HTTP server a convenient way to alter document's encoding according to content negotiation; certain HTTP server software can do it, for example Apache with the module mod_charset_lite. [2]

Second, a declaration can be included within the document itself.

For HTML it is possible to include this information inside the head element near the top of the document: [3] 

<metahttp-equiv="Content-Type"content="text/html; charset=utf-8">

HTML5 also allows the following syntax to mean exactly the same: [3] 

<metacharset="utf-8">

XHTML documents have a third option: to express the character encoding via XML declaration, as follows: [4] 

<?xml version="1.0" encoding="utf-8"?>

With this second approach, because the character encoding cannot be known until the declaration is parsed, there is a problem knowing which character encoding is used in the document up to and including the declaration itself. If the character encoding is an ASCII extension then the content up to and including the declaration itself should be pure ASCII and this will work correctly. For character encodings that are not ASCII extensions (i.e. not a superset of ASCII), such as UTF-16BE and UTF-16LE, a processor of HTML, such as a web browser, should be able to parse the declaration in some cases through the use of heuristics.

Encoding detection algorithm

As of HTML5 the recommended charset is UTF-8. [3] An "encoding sniffing algorithm" is defined in the specification to determine the character encoding of the document based on multiple sources of input, including:

  1. Explicit user instruction
  2. An explicit meta tag within the first 1024 bytes of the document
  3. A byte order mark (BOM) within the first three bytes of the document
  4. The HTTP Content-Type or other transport layer information
  5. Analysis of the document bytes looking for specific sequences or ranges of byte values, [5] and other tentative detection mechanisms.

Characters outside of the printable ASCII range (32 to 126) usually appear incorrectly. This presents few problems for English-speaking users, but other languages regularly—in some cases, always—require characters outside that range. In Chinese, Japanese, and Korean (CJK) language environments where there are several different multi-byte encodings in use, auto-detection is also often employed. Finally, browsers usually permit the user to override incorrect charset label manually as well.

It is increasingly common for multilingual websites and websites in non-Western languages to use UTF-8, which allows use of the same encoding for all languages. UTF-16 or UTF-32, which can be used for all languages as well, are less widely used because they can be harder to handle in programming languages that assume a byte-oriented ASCII superset encoding, and they are less efficient for text with a high frequency of ASCII characters, which is usually the case for HTML documents.

Successful viewing of a page is not necessarily an indication that its encoding is specified correctly. If the page's creator and reader are both assuming some platform-specific character encoding, and the server does not send any identifying information, then the reader will nonetheless see the page as the creator intended, but other readers on different platforms or with different native languages will not see the page as intended.

Permitted encodings

The WHATWG Encoding Standard, referenced by recent HTML standards (the current WHATWG HTML Living Standard, as well as the formerly competing W3C HTML 5.0 and 5.1) specifies a list of encodings which browsers must support. The HTML standards forbid support of other encodings. [6] [7] [8] The Encoding Standard further stipulates that new formats, new protocols (even when existing formats are used) and authors of new documents are required to use UTF-8 exclusively. [9]

Besides UTF-8, the following encodings are explicitly listed in the HTML standard itself, with reference to the Encoding Standard: [8]

  1. ↑ Also specified for TIS-620 , ISO-8859-11 and related labels. [9]
  2. ↑ Also specified for ASCII , ISO-8859-1 and related labels. [9]
  3. ↑ Also specified for ISO-8859-9 and related labels. [9]
  4. ↑ Specified with 0xA3A0 as a duplicate encoding of the ideographic space (U+3000) for compatibility reasons, and as such excluding U+E5E5 (a private use character). [10] [11] Also, specified with 0x80 accepted as an alternative encoding of the euro sign (U+20AC; see Windows-936). [12] Otherwise, follows the mappings from the 2005 standard. [11]
  5. ↑ Hong Kong Supplementary Character Set variant, [13] although most of the HKSCS extensions (those with lead bytes less than 0xA1) are not included by the encoder, only by the decoder. [14]
  6. ↑ The specification includes IBM and NEC extensions, [15] and is more precisely Windows-31J. [13]
  7. ↑ The specification uses the same index as used for Shift JIS (insofar as is within reach), i.e. includes NEC extensions. Half-width kana is converted to fullwidth by the encoder, [16] but accepted using an escape sequence (ESC 0x28 0x49) by the decoder. [17] Shift Out and Shift In (0x0E and 0x0F) are excluded entirely to prevent attacks. [17] [18]
  8. ↑ Actually Unified Hangul Code (Windows-949), which is a superset which covers the entire Hangul Syllables block. [13] [19]
  9. ↑ Specified for decoding only; form submissions from UTF-16-coded documents are to be encoded in UTF-8. [20]
  10. ↑ For compatibility with deployed content, also specified for the plain UTF-16 label, [21] although a byte order mark (BOM), if present, takes priority over any label. [22] Specified for decoding only; form submissions from UTF-16-coded documents are to be encoded in UTF-8. [20]
  11. ↑ Maps 0x00 through 0x7F to U+0000 through U+007F, and 0x80 through 0xFF to U+F780 through U+F7FF (a Private Use Area range), such that the low 8 bits of the code point always match the original byte. [23]

The following additional encodings are listed in the Encoding Standard, and support for them is therefore also required: [9]

  1. ↑ Uses the same encoder and decoder as ISO-8859-8, but is not subject to the visual-order behaviour which is used for documents labelled as ISO-8859-8. [24]
  2. ↑ Titled KOI8-U and specified for both KOI8-U and KOI8-RU labels; [9] follows KOI8-RU in positions 0xAE and 0xBE (i.e. includes Ў/ў) [25] [26] but KOI8-U in positions 0x93–9F. [25]
  3. ↑ Also specified for GB2312 and related labels. Handled the same as GB 18030 for decoding purposes. [27] For encoding purposes, labelling as GBK (or GB 2312) excludes four-byte codes, and favours the one-byte 0x80 representation for U+20AC. [10]
  4. ↑ The specification uses the same index as used for Shift JIS (insofar as is within reach of the EUC code set 1), i.e. includes NEC extensions. JIS X 0212 is included for decoding only. [28]

The following encodings are listed as explicit examples of forbidden encodings: [8]

The standard also defines a "replacement" decoder, which maps all content labelled as certain encodings to the replacement character (�), refusing to process it at all. This is intended to prevent attacks (e.g. cross site scripting) which may exploit a difference between the client and server in what encodings are supported in order to mask malicious content. [29] Although the same security concern applies to ISO-2022-JP and UTF-16, which also allow sequences of ASCII bytes to be interpreted differently, this approach was not seen as feasible for them since they are comparatively more frequently used in deployed content. [30] The following encodings receive this treatment: [31]

Character references

In addition to native character encodings, characters can also be encoded as character references, which can be numeric character references (decimal or hexadecimal) or character entity references. Character entity references are also sometimes referred to as named entities, or HTML entities for HTML. HTML's usage of character references derives from SGML.

HTML character references

A numeric character reference in HTML refers to a character by its Universal Character Set/Unicode code point , and uses the format

&#nnnn;

or

&#xhhhh;

where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form. The x must be lowercase in XML documents. The nnnn or hhhh may be any number of digits and may include leading zeros. The hhhh may mix uppercase and lowercase, though uppercase is the usual style.

Not all web browsers or email clients used by receivers of HTML documents, or text editors used by authors of HTML documents, will be able to render all HTML characters. Most modern software is able to display most or all of the characters for the user's language, and will draw a box or other clear indicator for characters they cannot render.

For codes from 0 to 127, the original 7-bit ASCII standard set, most of these characters can be used without a character reference. Codes from 160 to 255 can all be created using character entity names. Only a few higher-numbered codes can be created using entity names, but all can be created by decimal number character reference.

Character entity references can also have the format &name; where name is a case-sensitive alphanumeric string. For example, "λ" can also be encoded as &lambda; in an HTML document. The character entity references &lt;, &gt;, &quot; and &amp; are predefined in HTML and SGML, because <, >, " and & are already used to delimit markup. This notably did not include XML's &apos; (') entity prior to HTML5. For a list of all named HTML character entity references along with the versions in which they were introduced, see List of XML and HTML character entity references.

Unnecessary use of HTML character references may significantly reduce HTML readability. If the character encoding for a web page is chosen appropriately, then HTML character references are usually only required for markup delimiting characters as mentioned above, and for a few special characters (or none at all if a native Unicode encoding like UTF-8 is used). Incorrect HTML entity escaping may also open up security vulnerabilities for injection attacks such as cross-site scripting. If HTML attributes are left unquoted, certain characters, most importantly whitespace, such as space and tab, must be escaped using entities. Other languages related to HTML have their own methods of escaping characters.

XML character references

Unlike traditional HTML with its large range of character entity references, in XML there are only five predefined character entity references. These are used to escape characters that are markup sensitive in certain contexts: [32]

&amp;& ampersand U+0026
&lt;<less-than signU+003C
&gt;>greater-than signU+003E
&quot;"quotation markU+0022
&apos;'apostropheU+0027

All other character entity references have to be defined before they can be used. For example, use of &eacute; (which gives é, Latin lower-case E with acute accent, U+00E9 in Unicode) in an XML document will generate an error unless the entity has already been defined. XML also requires that the x in hexadecimal numeric references be in lowercase: for example &#xA1b rather than &#XA1b. XHTML, which is an XML application, supports the HTML entity set, along with XML's predefined entities.

See also

Related Research Articles

<span class="mw-page-title-main">Character encoding</span> Using numbers to represent text characters

Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values that make up a character encoding are known as "code points" and collectively comprise a "code space", a "code page", or a "character map".

<span class="mw-page-title-main">ISO/IEC 8859-1</span> Character encoding

ISO/IEC 8859-1:1998, Information technology—8-bit single-byte coded graphic character sets—Part 1: Latin alphabet No. 1, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. ISO/IEC 8859-1 encodes what it refers to as "Latin alphabet no. 1", consisting of 191 characters from the Latin script. This character-encoding scheme is used throughout the Americas, Western Europe, Oceania, and much of Africa. It is the basis for some popular 8-bit character sets and the first two blocks of characters in Unicode.

ISO/IEC 8859 is a joint ISO and IEC series of standards for 8-bit character encodings. The series of standards consists of numbered parts, such as ISO/IEC 8859-1, ISO/IEC 8859-2, etc. There are 15 parts, excluding the abandoned ISO/IEC 8859-12. The ISO working group maintaining this series of standards has been disbanded.

<span class="mw-page-title-main">Plain text</span> Term for computer data consisting only of unformatted characters of readable material

In computing, plain text is a loose term for data that represent only characters of readable material but not its graphical representation nor other objects. It may also include a limited number of "whitespace" characters that affect simple arrangement of text, such as spaces, line breaks, or tabulation characters. Plain text is different from formatted text, where style information is included; from structured text, where structural parts of the document such as paragraphs, sections, and the like are identified; and from binary files in which some portions must be interpreted as binary objects.

Web pages authored using HyperText Markup Language (HTML) may contain multilingual text represented with the Unicode universal character set. Key to the relationship between Unicode and HTML is the relationship between the "document character set", which defines the set of characters that may be present in an HTML document and assigns numbers to them, and the "external character encoding", or "charset", used to encode a given document as a sequence of bytes.

UTF-8 is a character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode Transformation Format – 8-bit. Almost every webpage is stored in UTF-8.

The byte-order mark (BOM) is a particular usage of the special Unicode character code, U+FEFFZERO WIDTH NO-BREAK SPACE, whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text:

<span class="mw-page-title-main">Mojibake</span> Garbled text as a result of incorrect character encodings

Mojibake is the garbled or gibberish text that is the result of text being decoded using an unintended character encoding. The result is a systematic replacement of symbols with completely unrelated ones, often from a different writing system.

<span class="mw-page-title-main">Windows-1252</span> Windows character set for Latin alphabet

Windows-1252 or CP-1252 is a legacy single-byte character encoding that is used by default in Microsoft Windows throughout the Americas, Western Europe, Oceania, and much of Africa.

ISO/IEC 8859-8, Information technology — 8-bit single-byte coded graphic character sets — Part 8: Latin/Hebrew alphabet, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings. ISO/IEC 8859-8:1999 from 1999 represents its second and current revision, preceded by the first edition ISO/IEC 8859-8:1988 in 1988. It is informally referred to as Latin/Hebrew. ISO/IEC 8859-8 covers all the Hebrew letters, but no Hebrew vowel signs. IBM assigned code page 916 to it. This character set was also adopted by Israeli Standard SI1311:2002, with some extensions.

ISO/IEC 8859-9:1999, Information technology — 8-bit single-byte coded graphic character sets — Part 9: Latin alphabet No. 5, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1989. It is designated ECMA-128 by Ecma International and TS 5881 as a Turkish standard. It is informally referred to as Latin-5 or Turkish. It was designed to cover the Turkish language, designed as being of more use than the ISO/IEC 8859-3 encoding. It is identical to ISO/IEC 8859-1 except for the replacement of six Icelandic characters with characters unique to the Turkish alphabet. And the uppercase of i is İ; the lowercase of I is ı.

ISO/IEC 2022Information technology—Character code structure and extension techniques, is an ISO/IEC standard in the field of character encoding. It is equivalent to the ECMA standard ECMA-35, the ANSI standard ANSI X3.41 and the Japanese Industrial Standard JIS X 0202. Originating in 1971, it was most recently revised in 1994.

Windows-1251 is an 8-bit character encoding, designed to cover languages that use the Cyrillic script such as Russian, Ukrainian, Belarusian, Bulgarian, Serbian Cyrillic, Macedonian and other languages.

A numeric character reference (NCR) is a common markup construct used in SGML and SGML-derived markup languages such as HTML and XML. It consists of a short sequence of characters that, in turn, represents a single character. Since WebSgml, XML and HTML 4, the code points of the Universal Character Set (UCS) of Unicode are used. NCRs are typically used in order to represent characters that are not directly encodable in a particular document. When the document is interpreted by a markup-aware reader, each NCR is treated as if it were the character it represents.

Windows code page 1253, commonly known by its IANA-registered name Windows-1253 or abbreviated as cp1253, is a Microsoft Windows code page used to write modern Greek. It is not capable of supporting the older polytonic Greek.

Windows-1254 is a code page used under Microsoft Windows, to write Turkish that it was designed for. Characters with codepoints A0 through FF are compatible with ISO 8859-9, but the CR range, which is reserved for C1 control codes in ISO 8859, is instead used for additional characters. It matches Windows-1252 except for the replacement of six Icelandic characters with characters unique to the Turkish alphabet.

Windows code pages are sets of characters or code pages used in Microsoft Windows from the 1980s and 1990s. Windows code pages were gradually superseded when Unicode was implemented in Windows, although they are still supported both within Windows and other platforms, and still apply when Alt code shortcuts are used.

ISO-8859-8-I is the IANA charset name for the character encoding ISO/IEC 8859-8 used together with the control codes from ISO/IEC 6429 for the C0 and C1 (80–9F) parts. The characters are in logical order.

Character encoding detection, charset detection, or code page detection is the process of heuristically guessing the character encoding of a series of bytes that represent text. The technique is recognised to be unreliable and is only used when specific metadata, such as a HTTP Content-Type: header is either not available, or is assumed to be untrustworthy.

Microsoft Windows code page 932, also called Windows-31J amongst other names, is the Microsoft Windows code page for the Japanese language, which is an extended variant of the Shift JIS Japanese character encoding. It contains standard 7-bit ASCII codes, and Japanese characters are indicated by the high bit of the first byte being set to 1. Some code points in this page require a second byte, so characters use either 8 or 16 bits for encoding.

References

  1. ↑ Fielding, R.; Reschke, J. (June 2014), "Content-Type", in Fielding, R; Reschke, J (eds.), Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content, IETF, doi:10.17487/RFC7231, S2CID   14399078 , retrieved 30 July 2014
  2. ↑ "Apache Module mod_charset_lite".
  3. 1 2 3 "Specifying the document's character encoding", HTML5, World Wide Web Consortium, 14 December 2017, retrieved 28 May 2018
  4. ↑ Bray, T.; Paoli, J.; Sperberg-McQueen, C.; Maler, E.; Yergeau, F. (26 November 2008), "Prolog and Document Type Declaration", XML, W3C , retrieved 8 March 2010
  5. ↑ "HTML5 prescan a byte stream to determine its encoding".
  6. ↑ "8.2.2.3. Character encodings". HTML 5.1 Standard. W3C.
  7. ↑ "8.2.2.3. Character encodings". HTML 5 Standard. W3C.
  8. 1 2 3 "12.2.3.3 Character encodings". HTML Living Standard. WHATWG.
  9. 1 2 3 4 5 6 van Kesteren, Anne. "4.2: Names and labels". Encoding Standard. WHATWG.
  10. 1 2 van Kesteren, Anne. "10.2.2. gb18030 encoder". Encoding Standard. WHATWG.
  11. 1 2 van Kesteren, Anne. "5. Indexes (§ index gb18030)". Encoding Standard. WHATWG.
  12. ↑ van Kesteren, Anne. "10.2.1. gb18030 decoder". Encoding Standard. WHATWG.
  13. 1 2 3 Mozilla Foundation. "Notable Differences from IANA Naming". Crate encoding_rs. docs.rs.
  14. ↑ van Kesteren, Anne. "5. Indexes (§ index Big5 pointer)". Encoding Standard. WHATWG.
  15. ↑ van Kesteren, Anne. "5. Indexes (§ Index jis0208)". Encoding Standard. WHATWG.
  16. ↑ van Kesteren, Anne. "5. Indexes (§ Index ISO-2022-JP katakana)". Encoding Standard. WHATWG.
  17. 1 2 van Kesteren, Anne. "12.2.1. ISO-2022-JP decoder". Encoding Standard. WHATWG.
  18. ↑ van Kesteren, Anne. "12.2.2. ISO-2022-JP encoder". Encoding Standard. WHATWG.
  19. ↑ van Kesteren, Anne. "5. Indexes (§ index EUC-KR)". Encoding Standard. WHATWG.
  20. 1 2 van Kesteren, Anne. "4.3. Output encodings". Encoding Standard. WHATWG.
  21. ↑ van Kesteren, Anne. "14.4. UTF-16LE". Encoding Standard. WHATWG.
  22. ↑ van Kesteren, Anne. "6. Hooks for standards (§ decode)". Encoding Standard. WHATWG.
  23. ↑ van Kesteren, Anne. "14.5. x-user-defined". Encoding Standard. WHATWG.
  24. ↑ van Kesteren, Anne. "9. Legacy single-byte encodings (§ Note)". Encoding Standard. WHATWG.
  25. 1 2 van Kesteren, Anne. "index KOI8-U visualization". Encoding Standard. WHATWG.
  26. ↑ "Bug 17053: Support KOI8-RU mapping for KOI8-U". W3C Bugzilla. 19 August 2015.
  27. ↑ van Kesteren, Anne. "10.1. GBK". Encoding Standard. WHATWG.
  28. ↑ van Kesteren, Anne. "5. Indexes (§ Index jis0212)". Encoding Standard. WHATWG.
  29. ↑ van Kesteren, Anne. "14.1: replacement". Encoding Standard. WHATWG.
  30. ↑ van Kesteren, Anne. "2: Security background". Encoding Standard. WHATWG.
  31. ↑ van Kesteren, Anne. "4.2: Names and labels (§ replacement)". Encoding Standard. WHATWG.
  32. ↑ Bray, T.; Paoli, J.; Sperberg-McQueen, C.; Maler, E.; Yergeau, F. (26 November 2008), "Character and Entity References", XML, W3C , retrieved 8 March 2010
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.