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Unicode and HTML

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Unicode and HTML
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{{Rewrite|date=July 2018}}{{Multiple issues|{{primary sources|date=December 2011}}{{essay-like|date=December 2011}}{{refimprove|date=January 2011}}}}{{SpecialChars}}{{Html series}}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 a 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.In RFC 1866, the initial HTML 2.0 standard, the document character set was defined as ISO-8859-1. It was extended to ISO 10646 (which is basically equivalent to Unicode) by RFC 2070. It does not vary between documents of different languages or created on different platforms. The external character encoding is chosen by the author of the document (or the software the author uses to create the document) and determines how the bytes used to store and/or transmit the document map to characters from the document character set. Characters not present in the chosen external character encoding may be represented by character entity references.The relationship between Unicode and HTML tends to be a difficult topic for many computer professionals, document authors, and web users alike. The accurate representation of text in web pages from different natural languages and writing systems is complicated by the details of character encoding, markup language syntax, font, and varying levels of support by web browsers.

HTML document characters

Web pages are typically HTML or XHTML documents. Both types of documents consist, at a fundamental level, of characters, which are graphemes and grapheme-like units, independent of how they manifest in computer storage systems and networks.An HTML document is a sequence of Unicode characters. More specifically, HTML 4.0 documents are required to consist of characters in the HTML document character set : a character repertoire wherein each character is assigned a unique, non-negative integer code point. This set is defined in the HTML 4.0 DTD, which also establishes the syntax (allowable sequences of characters) that can produce a valid HTML document. The HTML document character set for HTML 4.0 consists of most, but not all, of the characters jointly defined by Unicode and ISO/IEC 10646: the Universal Character Set (UCS).Like HTML documents, an XHTML document is a sequence of Unicode characters. However, an XHTML document is an XML document, which, while not having an explicit "document character" layer of abstraction, nevertheless relies upon a similar definition of permissible characters that cover most, but not all, of the Unicode/UCS character definitions. The sets used by HTML and XHTML/XML are slightly different, but these differences have little effect on the average document author.Regardless of whether the document is HTML or XHTML, when stored on a file system or transmitted over a network, the document's characters are encoded as a sequence of bit octets (bytes) according to a particular character encoding. This encoding may either be a Unicode Transformation Format, like UTF-8, that can directly encode any Unicode character, or a legacy encoding, like Windows-1252, that cannot. However, even when using encodings that do not support all Unicode characters, the encoded document may make use of numeric character references. For example, ☺ (☺) is used to indicate a smiling face character in the Unicode character set.

Character encoding

In order to support all Unicode characters without resorting to numeric character references, a web page must have an encoding covering all of Unicode. The most popular is UTF-8, where the ASCII characters, such as English letters, digits, and some other common characters are preserved unchanged against ASCII. This makes HTML code (such as
and
) unchanged compared to ASCII. Characters outside the ASCII range are stored in 2-4 bytes. It is also possible to use UTF-16 where most characters are stored as two bytes with varying endianness, which is supported by modern browsers but less commonly used.

Numeric character references

In order to work around the limitations of legacy encodings, HTML is designed such that it is possible to represent characters from the whole of Unicode inside an HTML document by using a numeric character reference: a sequence of characters that explicitly spell out the Unicode code point of the character being represented. A character reference takes the form &#N;, where N is either a decimal number for the Unicode code point, or a hexadecimal number, in which case it must be prefixed by x. The characters that compose the numeric character reference are universally representable in every encoding approved for use on the Internet.For example, a Unicode code point like U+5408, which corresponds to a particular Chinese character, has to be converted to a decimal number, preceded by &# and followed by ;, like this: 合, which produces this: 合 (if it doesn't look like a Chinese character, see ).The support for hexadecimal in this context is more recent, so older browsers might have problems displaying characters referenced with hexadecimal numbers—but they will probably have a problem displaying Unicode characters above code point 255 anyway. To ensure better compatibility with older browsers, it is still a common practice to convert the hexadecimal code point into a decimal value (for example 合 instead of 合).

Named character entities

In HTML 4, there is a standard set of 252 named character entities for characters - some common, some obscure - that are either not found in certain character encodings or are markup sensitive in some contexts (for example angle brackets and quotation marks). Although any Unicode character can be referenced by its numeric code point, some HTML document authors prefer to use these named entities instead, where possible, as they are less cryptic and were better supported by early browsers.Character entities can be included in an HTML document via the use of entity references, which take the form &EntityName;, where EntityName is the name of the entity. For example, —, much like — or —, represents {{U+|2014}}: the em dash character "—" even if the character encoding used doesn't contain that character.For the full list, see: List of XML and HTML character entity references.

Character encoding determination

In order to correctly process HTML, a web browser must ascertain which Unicode characters are represented by the encoded form of an HTML document. In order to do this, the web browser must know what encoding was used.

Encoding information

When a document is transmitted via a MIME message or a transport that uses MIME content types such as an HTTP response, the message may signal the encoding via a Content-Type header, such as Content-Type: text/html; charset=UTF-8. Other external means of declaring encoding are permitted but rarely used. If the document uses a Unicode encoding, the encoding info might also be present in the form of a Byte order mark. Finally, the encoding can be declared via the HTML syntax. For the text/html serialisation then, as long as the page is encoded in an extension of ASCII (such as UTF-8, and thus, not if the page is using UTF-16), a meta element, like or (starting with HTML5) can be used. For HTML pages serialized as XML, then declaration options is to either rely on the encoding default (which for XML documents is UTF-8), or to use an XML encoding declaration. The meta attribute plays no role in HTML served as XML.

Encoding defaults

An encoding default applies when there is no external or internal encoding declaration and also no Byte order mark. While the encoding default for HTML pages served as XML is required to be UTF-8, the encoding default for a regular Web page (that is: for HTML pages serialized as text/html) varies depending on the localization of the browser. For a system set up mainly for Western European languages, it will generally be Windows-1252. For Cyrillic alphabet locales, the default is typically Windows-1251. For a browser from a location where legacy multi-byte character encodings are prevalent, some form of auto-detection is likely to be applied.

Encoding trends

Because of the legacy of 8-bit text representations in programming languages and operating systems and the desire to avoid burdening users with the need to understand the nuances of encoding, many text editors used by HTML authors are unable or unwilling to offer a choice of encodings when saving files to disk and often do not even allow input of characters beyond a very limited range. Consequently, many HTML authors are unaware of encoding issues and may not have any idea what encoding their documents actually use. Misunderstandings, such as the belief that the encoding declaration affects a change in the actual encoding (whereas it is actually just a label that could be inaccurate), is also a reason for this editor attitude. Another factor contributing in the same direction, is the arrival of UTF-8 — which greatly diminishes the need for other encodings, and thus modern editors tends to default, as recommended by the HTML5 specification,WEB,weblink HTML5, Ian Hickson, 17 September 2011, 2011, Authors are encouraged to use UTF-8. Conformance checkers may advise authors against using legacy encodings. [RFC3629] Authoring tools should default to using UTF-8 for newly created documents. [RFC3629], to UTF-8.

Byte order mark/Unicode sniffing

For both serializations of HTML (content-type "text/html" and content/type "application/xhtml+xml"), the Byte order mark (BOM) is an effective way to transmit encoding information within an HTML document. For UTF-8, the BOM is optional, while it is a must for the UTF-16 and the UTF-32 encodings. (Note: UTF-16 and UTF-32 without the BOM are formally known under different names, they are different encodings, and thus needs some form of encoding declaration – see UTF-16BE, UTF-16LE, UTF-32LE and UTF-32BE.) The use of the BOM character (U+FEFF) means that the encoding automatically declares itself to any processing application. Processing applications need only look for an initial 0x0000FEFF, 0xFEFF or 0xEFBBBF in the byte stream to identify the document as UTF-32, UTF-16 or UTF-8 encoded respectively. No additional metadata mechanisms are required for these encodings since the byte-order mark includes all of the information necessary for processing applications. In most circumstances the byte-order mark character is handled by editing applications separately from the other characters so there is little risk of an author removing or otherwise changing the byte order mark to indicate the wrong encoding (as can happen when the encoding is declared in English/Latin script). If the document lacks a byte-order mark, the fact that the first non-blank printable character in an HTML document is supposed to be "

- content above as imported from Wikipedia
- "Unicode and HTML" does not exist on GetWiki (yet)
- time: 5:04am EST - Sun, Nov 18 2018
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