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git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@638 48356398-32a2-884e-a903-53898d9a118a
446 lines
20 KiB
HTML
446 lines
20 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<meta name="description" content="Describes the rationale for using UTF-8, the ramifications otherwise, and how to make the switch." />
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<link rel="stylesheet" type="text/css" href="./style.css" />
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<script defer="defer" type="text/javascript" src="./toc-gen.js"></script>
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<style type="text/css">
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.minor td {font-style:italic;}
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</style>
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<title>UTF-8 - HTML Purifier</title>
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<!-- Note to users: this document, though professing to be UTF-8, attempts
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to use only ASCII characters, because most webservers are configured
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to send HTML as ISO-8859-1 -->
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</head><body>
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<h1>UTF-8</h1>
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<div id="filing">Filed under End-User</div>
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<div id="index">Return to the <a href="index.html">index</a>.</div>
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<p>Character encoding and character sets, in truth, are not that
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difficult to understand. But if you don't understand them, you are going
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to be caught by surprise by some of HTML Purifier's behavior, namely
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the fact that it operates UTF-8 or the limitations of the character
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encoding transformations it does. This document will walk you through
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determining the encoding of your system and how you should handle
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this information. It will stay away from excessive discussion on
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the internals of character encoding, but offer the information in
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asides that can easily be skipped.</p>
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<blockquote class="aside">
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<div class="label">Asides</div>
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<p>Text in this formatting is an <strong>aside</strong>,
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interesting tidbits for the curious but not strictly necessary material to
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do the tutorial. If you read this text, you'll come out
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with a greater understanding of the underlying issues.</p>
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</blockquote>
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<h2 id="findcharset">Finding the real encoding</h2>
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<p>In the beginning, there was ASCII, and things were simple. But they
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weren't good, for no one could write in Cryllic or Thai. So there
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exploded a proliferation of character encodings to remedy the problem
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by extending the characters ASCII could express. This ridiculously
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simplified version of the history of character encodings shows us that
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there are now many character encodings floating around.</p>
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<blockquote class="aside">
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<p>A <strong>character encoding</strong> tells the computer how to
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interpret raw zeroes and ones into real characters. It
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usually does this by pairing numbers with characters.</p>
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<p>There are many different types of character encodings floating
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around, but the ones we deal most frequently with are ASCII,
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8-bit encodings, and Unicode-based encodings.</p>
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<ul>
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<li><strong>ASCII</strong> is a 7-bit encoding based on the
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English alphabet.</li>
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<li><strong>8-bit encodings</strong> are extensions to ASCII
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that add a potpourri of useful, non-standard characters
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like é and æ. They can only add 127 characters,
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so usually only support one script at a time. When you
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see a page on the web, chances are it's encoded in one
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of these encodings.</li>
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<li><strong>Unicode-based encodings</strong> implement the
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Unicode standard and include UTF-8, UCS-2 and UTF-16.
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They go beyond 8-bits (the first two are variable length,
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while the second one uses 16-bits), and support almost
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every language in the world. UTF-8 is gaining traction
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as the dominant international encoding of the web.</li>
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</ul>
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</blockquote>
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<p>The first step of our journey is to find out what the encoding of
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<em>insert-application-here</em> is. The most reliable way is to ask your
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browser:</p>
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<dl>
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<dt>Mozilla Firefox</dt>
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<dd>Tools > Page Info: Encoding</dd>
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<dt>Internet Explorer</dt>
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<dd>View > Encoding: bulleted item is unofficial name</dd>
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</dl>
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<p>Internet Explorer won't give you the mime (i.e. useful/real) name of the
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character encoding, so you'll have to look it up using their description.
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Some common ones:</p>
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<table class="table">
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<thead><tr>
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<th>IE's Description</th>
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<th>Mime Name</th>
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</tr></thead>
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<tbody>
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<tr><th colspan="2">Windows</th></tr>
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<tr><td>Arabic (Windows)</td><td>Windows-1256</td></tr>
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<tr><td>Baltic (Windows)</td><td>Windows-1257</td></tr>
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<tr><td>Central European (Windows)</td><td>Windows-1250</td></tr>
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<tr><td>Cyrillic (Windows)</td><td>Windows-1251</td></tr>
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<tr><td>Greek (Windows)</td><td>Windows-1253</td></tr>
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<tr><td>Hebrew (Windows)</td><td>Windows-1255</td></tr>
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<tr><td>Thai (Windows)</td><td>TIS-620</td></tr>
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<tr><td>Turkish (Windows)</td><td>Windows-1254</td></tr>
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<tr><td>Vietnamese (Windows)</td><td>Windows-1258</td></tr>
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<tr><td>Western European (Windows)</td><td>Windows-1252</td></tr>
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</tbody>
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<tbody>
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<tr><th colspan="2">ISO</th></tr>
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<tr><td>Arabic (ISO)</td><td>ISO-8859-6</td></tr>
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<tr><td>Baltic (ISO)</td><td>ISO-8859-4</td></tr>
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<tr><td>Central European (ISO)</td><td>ISO-8859-2</td></tr>
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<tr><td>Cyrillic (ISO)</td><td>ISO-8859-5</td></tr>
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<tr class="minor"><td>Estonian (ISO)</td><td>ISO-8859-13</td></tr>
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<tr class="minor"><td>Greek (ISO)</td><td>ISO-8859-7</td></tr>
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<tr><td>Hebrew (ISO-Logical)</td><td>ISO-8859-8-l</td></tr>
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<tr><td>Hebrew (ISO-Visual)</td><td>ISO-8859-8</td></tr>
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<tr class="minor"><td>Latin 9 (ISO)</td><td>ISO-8859-15</td></tr>
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<tr class="minor"><td>Turkish (ISO)</td><td>ISO-8859-9</td></tr>
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<tr><td>Western European (ISO)</td><td>ISO-8859-1</td></tr>
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</tbody>
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<tbody>
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<tr><th colspan="2">Other</th></tr>
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<tr><td>Chinese Simplified (GB18030)</td><td>GB18030</td></tr>
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<tr><td>Chinese Simplified (GB2312)</td><td>GB2312</td></tr>
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<tr><td>Chinese Simplified (HZ)</td><td>HZ</td></tr>
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<tr><td>Chinese Traditional (Big5)</td><td>Big5</td></tr>
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<tr><td>Japanese (Shift-JIS)</td><td>Shift_JIS</td></tr>
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<tr><td>Japanese (EUC)</td><td>EUC-JP</td></tr>
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<tr><td>Korean</td><td>EUC-KR</td></tr>
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<tr><td>Unicode (UTF-8)</td><td>UTF-8</td></tr>
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</tbody>
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</table>
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<p>Internet Explorer does not recognize some of the more obscure
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character encodings, and having to lookup the real names with a table
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is a pain, so I recommend using Mozilla Firefox to find out your
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character encoding.</p>
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<h2 id="findmetacharset">Finding the embedded encoding</h2>
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<p>At this point, you may be asking, "Didn't we already find out our
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encoding?" Well, as it turns out, there are multiple places where
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a web developer can specify a character encoding, and one such place
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is in a <code>META</code> tag:</p>
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<pre><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /></pre>
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<p>You'll find this in the <code>HEAD</code> section of an HTML document.
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The text to the right of <code>charset=</code> is the "claimed"
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encoding: the HTML claims to be this encoding, but whether or not this
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is actually the case depends on other factors. For now, take note
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if your <code>META</code> tag claims that either:</p>
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<ol>
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<li>The character encoding is the same as the one reported by the
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browser,</li>
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<li>The character encoding is different from the browser's, or</li>
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<li>There is no <code>META</code> tag at all! (horror, horror!)</li>
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</ol>
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<h2 id="fixcharset">Fixing the encoding</h2>
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<p>If your <code>META</code> encoding and your real encoding match,
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savvy! You can skip this section. If they don't...</p>
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<h3 id="fixcharset-none">No embedded encoding</h3>
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<p>If this is the case, you'll want to add in the appropriate
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<code>META</code> tag to your website. It's as simple as copy-pasting
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the code snippet above and replacing UTF-8 with whatever is the mime name
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of your real encoding.</p>
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<blockquote class="aside">
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<p>For all those skeptics out there, there is a very good reason
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why the character encoding should be explicitly stated. When the
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browser isn't told what the character encoding of a text is, it
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has to guess: and sometimes the guess is wrong. Hackers can manipulate
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this guess in order to slip XSS pass filters and then fool the
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browser into executing it as active code. A great example of this
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is the <a href="http://shiflett.org/archive/177">Google UTF-7
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exploit</a>.</p>
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<p>You might be able to get away with not specifying a character
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encoding with the <code>META</code> tag as long as your webserver
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sends the right Content-Type header, but why risk it? Besides, if
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the user downloads the HTML file, there is no longer any webserver
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to define the character encoding.</p>
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</blockquote>
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<h3 id="fixcharset-diff">Embedded encoding disagrees</h3>
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<p>This is an extremely common mistake: another source is telling
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the browser what the
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character encoding is and is overriding the embedded encoding. This
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source usually is the Content-Type HTTP header that the webserver (i.e.
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Apache) sends. A usual Content-Type header sent with a page might
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look like this:</p>
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<pre>Content-Type: text/html; charset=ISO-8859-1</pre>
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<p>Notice how there is a charset parameter: this is the webserver's
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way of telling a browser what the character encoding is, much like
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the <code>META</code> tags we touched upon previously.</p>
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<blockquote class="aside"><p>In fact, the <code>META</code> tag is
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designed as a substitute for the HTTP header for contexts where
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sending headers is impossible (such as locally stored files without
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a webserver). Thus the name <code>http-equiv</code> (HTTP equivalent).
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</p></blockquote>
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<p>There are two ways to go about fixing this: changing the <code>META</code>
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tag to match the HTTP header, or changing the HTTP header to match
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the <code>META</code> tag. How do we know which to do? It depends
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on the website's content: after all, headers and tags are only ways of
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describing the actual characters on the web page.</p>
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<p>If your website:</p>
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<dl>
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<dt>...only uses ASCII characters,</dt>
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<dd>Either way is fine, but I recommend switching both to
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UTF-8 (more on this later).</dd>
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<dt>...uses special characters, and they display
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properly,</dt>
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<dd>Change the embedded encoding to the server encoding.</dd>
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<dt>...uses special characters, but users often complain that
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they come out garbled,</dt>
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<dd>Change the server encoding to the embedded encoding.</dd>
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</dl>
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<p>Changing a META tag is easy: just swap out the old encoding
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for the new. Changing the server (HTTP header) encoding, however,
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is slightly more difficult.</p>
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<h3 id="fixcharset-server">Changing the server encoding</h3>
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<h4 id="fixcharset-server-php">PHP header() function</h4>
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<p>The simplest way to handle this problem is to send the encoding
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yourself, via your programming language. Since you're using HTML
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Purifier, I'll assume PHP, although it's not too difficult to do
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similar things in
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<a href="http://www.w3.org/International/O-HTTP-charset#scripting">other
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languages</a>. The appropriate code is:</p>
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<pre><a href="http://php.net/header">header</a>('Content-Type:text/html; charset=UTF-8');</pre>
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<p>...replacing UTF-8 with whatever your embedded encoding is.
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This code must come before any output, so be careful about
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stray whitespace in your application.</p>
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<h4 id="fixcharset-server-nophp">Non-PHP</h4>
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<p>You may, for whatever reason, may need to set the character encoding
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on non-PHP files, usually plain ol' HTML files. Doing this
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is more of a hit-or-miss process: depending on the software being
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used as a webserver and the configuration of that software, certain
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techniques may work, or may not work.</p>
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<h4 id="fixcharset-server-htaccess">.htaccess</h4>
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<p>On Apache, you can use an .htaccess file to change the character
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encoding. I'll defer to
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<a href="http://www.w3.org/International/questions/qa-htaccess-charset">W3C</a>
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for the in-depth explanation, but it boils down to creating a file
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named .htaccess with the contents:</p>
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/mod_mime.html#addcharset">AddCharset</a> UTF-8 .html</pre>
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<p>Where UTF-8 is replaced with the character encoding you want to
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use and .html is a file extension that this will be applied to. This
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character encoding will then be set for any file directly in
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or in the subdirectories of directory you place this file in.</p>
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<p>If you're feeling particularly courageous, you can use:</p>
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> UTF-8</pre>
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<p>...which changes the character set Apache adds to any document that
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doesn't have any Content-Type parameters. This directive, which the
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default configuration file sets to iso-8859-1 for security
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reasons, is probably why your headers mismatch
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with the <code>META</code> tag. If you would prefer Apache not to be
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butting in on your character encodings, you can tell it not
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to send anything at all:</p>
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<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> Off</pre>
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<p>...making your <code>META</code> tags the sole source of
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character encoding information. In these cases, it is
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<em>especially</em> important to make sure you have valid <code>META</code>
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tags on your pages and all the text before them is ASCII.</p>
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<blockquote class="aside"><p>These directives can also be
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placed in httpd.conf file for Apache, but
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in most shared hosting situations you won't be able to edit this file.
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</p></blockquote>
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<h4 id="fixcharset-server-ext">File extensions</h4>
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<p>If you're not allowed to use .htaccess files, you can often
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piggy-back off of Apache's default AddCharset declarations to get
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your files in the proper extension. Here are Apache's default
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character set declarations:</p>
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<table class="table">
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<thead><tr>
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<th>Charset</th>
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<th>File extension(s)</th>
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</tr></thead>
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<tbody>
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<tr><td>ISO-8859-1</td><td>.iso8859-1 .latin1</td></tr>
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<tr><td>ISO-8859-2</td><td>.iso8859-2 .latin2 .cen</td></tr>
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<tr><td>ISO-8859-3</td><td>.iso8859-3 .latin3</td></tr>
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<tr><td>ISO-8859-4</td><td>.iso8859-4 .latin4</td></tr>
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<tr><td>ISO-8859-5</td><td>.iso8859-5 .latin5 .cyr .iso-ru</td></tr>
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<tr><td>ISO-8859-6</td><td>.iso8859-6 .latin6 .arb</td></tr>
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<tr><td>ISO-8859-7</td><td>.iso8859-7 .latin7 .grk</td></tr>
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<tr><td>ISO-8859-8</td><td>.iso8859-8 .latin8 .heb</td></tr>
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<tr><td>ISO-8859-9</td><td>.iso8859-9 .latin9 .trk</td></tr>
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<tr><td>ISO-2022-JP</td><td>.iso2022-jp .jis</td></tr>
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<tr><td>ISO-2022-KR</td><td>.iso2022-kr .kis</td></tr>
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<tr><td>ISO-2022-CN</td><td>.iso2022-cn .cis</td></tr>
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<tr><td>Big5</td><td>.Big5 .big5 .b5</td></tr>
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<tr><td>WINDOWS-1251</td><td>.cp-1251 .win-1251</td></tr>
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<tr><td>CP866</td><td>.cp866</td></tr>
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<tr><td>KOI8-r</td><td>.koi8-r .koi8-ru</td></tr>
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<tr><td>KOI8-ru</td><td>.koi8-uk .ua</td></tr>
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<tr><td>ISO-10646-UCS-2</td><td>.ucs2</td></tr>
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<tr><td>ISO-10646-UCS-4</td><td>.ucs4</td></tr>
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<tr><td>UTF-8</td><td>.utf8</td></tr>
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<tr><td>GB2312</td><td>.gb2312 .gb </td></tr>
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<tr><td>utf-7</td><td>.utf7</td></tr>
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<tr><td>EUC-TW</td><td>.euc-tw</td></tr>
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<tr><td>EUC-JP</td><td>.euc-jp</td></tr>
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<tr><td>EUC-KR</td><td>.euc-kr</td></tr>
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<tr><td>shift_jis</td><td>.sjis</td></tr>
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</tbody>
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</table>
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<p>So, for example, a file named <code>page.utf8.html</code> or
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<code>page.html.utf8</code> will probably be sent with the UTF-8 charset
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attached, the difference being that if there is an
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<code>AddCharset charset .html</code> declaration, it will override
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the .utf8 extension in <code>page.utf8.html</code> (precedence moves
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from right to left). By default, Apache has no such declaration.</p>
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<h4 id="fixcharset-server-iis">Microsoft IIS</h4>
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<p>If anyone can contribute information on how to configure Microsoft
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IIS to change character encodings, I'd be grateful.</p>
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<h3 id="fixcharset-xml">XML</h3>
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<p><code>META</code> tags are the most common source of embedded
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encodings, but they can also come from somewhere else: XML
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processing instructions. They look like:</p>
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<pre><?xml version="1.0" encoding="UTF-8"?></pre>
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<p>...and are most often found in XML documents (including XHTML).</p>
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<p>For XHTML, this processing instruction theoretically
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overrides the <code>META</code> tag. In reality, this happens only when the
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XHTML is actually served as legit XML and not HTML, which is almost
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always never due to Internet Explorer's lack of support for
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<code>application/xhtml+xml</code> (even though doing so is often
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argued to be <a href="http://www.hixie.ch/advocacy/xhtml">good practice</a>).</p>
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<p>For XML, however, this processing instruction is extremely important.
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Since most webservers are not configured to send charsets for .xml files,
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this is the only thing a parser has to go on. Furthermore, the default
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for XML files is UTF-8, which often butts heads with more common
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ISO-8859-1 encoding (you see this in garbled RSS feeds).</p>
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<p>In short, if you use XHTML and have gone through the
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trouble of adding the XML header, be sure to make sure it jives
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with your <code>META</code> tags and HTTP headers.</p>
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<h3>Inside the process</h3>
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<p>This section is not required reading,
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but may answer some of your questions on what's going on in all
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this character encoding hocus pocus. If you're interested in
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|
moving on to the next phase, skip this section.</p>
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|
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<p>A logical question that follows all of our wheeling and dealing
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|
with multiple sources of character encodings is "Why are there
|
|
so many options?" To answer this question, we have to turn
|
|
back our definition of character encodings: they allow a program
|
|
to interpret bytes into human-readable characters.</p>
|
|
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|
<p>Thus, a chicken-egg problem: a character encoding
|
|
is necessary to interpret the
|
|
text of a document. A <code>META</code> tag is in the text of a document.
|
|
The <code>META</code> tag gives the character encoding. How can we
|
|
determine the contents of a <code>META</code> tag, inside the text,
|
|
if we don't know it's character encoding? And how do we figure out
|
|
the character encoding, if we don't know the contents of the
|
|
<code>META</code> tag?</p>
|
|
|
|
<p>Fortunantely for us, the characters we need to write the
|
|
<code>META</code> are in ASCII, which is pretty much universal
|
|
over every character encoding that is in common use today. So,
|
|
all the web-browser has to do is parse all the way down until
|
|
it gets to the Content-Type tag, extract the character encoding
|
|
tag, then re-parse the document according to this new information.</p>
|
|
|
|
<p>Obviously this is complicated, so browsers prefer the simpler
|
|
and more efficient solution: get the character encoding from a
|
|
somewhere other than the document itself, i.e. the HTTP headers,
|
|
much to the chagrin of HTML authors who can't set these headers.</p>
|
|
|
|
<h2 id="whyutf8">Why UTF-8?</h2>
|
|
|
|
<p>So, you've gone through all the trouble of ensuring that...</p>
|
|
|
|
<blockquote class="aside"><p>Needs completion!</p></blockquote>
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|
|
|
<h2 id="externallinks">Further Reading</h2>
|
|
|
|
<p>Many other developers have already discussed the subject of Unicode,
|
|
UTF-8 and internationalization, and I would like to defer to them for
|
|
a more in-depth look into character sets and encodings.</p>
|
|
|
|
<ul>
|
|
<li><a href="http://www.joelonsoftware.com/articles/Unicode.html">
|
|
The Absolute Minimum Every Software Developer Absolutely,
|
|
Positively Must Know About Unicode and Character Sets
|
|
(No Excuses!)</a> by Joel Spolsky, provides a <em>very</em>
|
|
good high-level look at Unicode and character sets in general.</li>
|
|
<li><a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8 on Wikipedia</a>,
|
|
provides a lot of useful details into the innards of UTF-8, although
|
|
it may be a little off-putting to people who don't know much
|
|
about Unicode to begin with.</li>
|
|
<li><a href="http://ppewww.physics.gla.ac.uk/~flavell/charset/form-i18n.html">
|
|
<code>FORM</code> submission and i18n</a> by A.J. Flavell,
|
|
discusses the pitfalls of attempting to create an internationalized
|
|
application without using UTF-8.</li>
|
|
</ul>
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</body>
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</html> |