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The document character set for XML and HTML 4.0 is Unicode (aka ISO 10646). This means that HTML browsers and XML processors should behave as if they used Unicode internally. But it doesn't mean that documents have to be transmitted in Unicode. As long as client and server agree on the encoding, they can use any encoding that can be converted to Unicode. Read more about the document character set.
It is very important that the character encoding of any XML or (X)HTML document is clearly labeled, so that clients can easily map these encodings to Unicode. This can be done in the following ways...
Send the 'charset' parameter in the Content-Type header of HTTP. Example:
Content-Type: text/html; charset=utf-8
To do this you will need to have access to server settings or serve your document via scripting (see Setting the HTTP charset parameter for more information).
For XML (including XHTML), use the encoding pseudo-attribute in the XML declaration at the start of a document or the text declaration at the start of an entity. Example:
<?xml version="1.0" encoding="utf-8" ?>
There are potential issues you should be aware of when using this with XHTML 1.0 served as HTML.
For HTML or XHTML served as HTML, you should always use the
<meta> tag inside
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" >
For XHTML, you need a slash at the end:
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
For a discussion of which approach is best for which type of (X)HTML document, see the tutorial Character sets & encodings in XHTML, HTML and CSS.
The examples above show declarations for
UTF-8 encoded content. This is likely to be the best choice of encoding for most
purposes, but it is not the only possibility.
If not using UTF-8 you should replace the
utf-8 text in the examples above with the name of the encoding you have chosen.
You can see the full list of character encoding names registered by IANA (long). In
practice, a few encodings will be preferred, most likely:
UTF-16, the other
encodings in the ISO-8859 series,
euc-kr, and so on.
It is important to not only use the encoding declarations above in HTTP or content, but also:
Save your data in the appropriate encoding from your editing environment.
Ensure that there is no conflict between what you declare in the document and what the server automatically applies, since server settings override in-document declarations.
For more information on these topics follow the links in Changing (X)HTML page encoding to UTF-8. Although it is written from a UTF-8 perspective, it applies to whatever encoding you use.
Values for the encoding attribute can be found in the IANA registry. Note that these are called charset names, although in reality they refer to the encodings, not the character sets.
If you want in-depth information related to the term 'charset', see an article by Dan Connolly ("Character Set" Considered Harmful) and a response by Glenn Adams (Character Set Terminology, SC2 vs. SC18 vs. Internet Standards).
Historic note: Rick Jellife proposed to use the SPREAD entities from ERCS.
References in specifications:
Interesting test pages: The 10th Unicode Conference
Characters and encodings in the Topic index
Characters and encodings in the Techniques index