What is CDATA in HTML? [duplicate]

2019-01-03 04:21发布

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What is the use of CDATA inside JavaScript tags and HTML?

<script type="text/javascript"> 
// <![CDATA[

// ]]>
</script>

6条回答
ら.Afraid
2楼-- · 2019-01-03 05:03

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.

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太酷不给撩
3楼-- · 2019-01-03 05:16

CDATA has no meaning at all in HTML.

CDATA is an XML construct which sets a tag's contents that is normally #PCDATA - parsed character data, to be instead taken as #CDATA, that is, non-parsed character data. It is only relevant and valid in XHTML.

It is used in script tags to avoid parsing < and &. In HTML, this is not needed, because in HTML, script is already #CDATA.

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地球回转人心会变
4楼-- · 2019-01-03 05:16

From http://en.wikipedia.org/wiki/CDATA:

Since it is useful to be able to use less-than signs (<) and ampersands (&) in web page scripts, and to a lesser extent styles, without having to remember to escape them, it is common to use CDATA markers around the text of inline and elements in XHTML documents. But so that the document can also be parsed by HTML parsers, which do not recognise the CDATA markers, the CDATA markers are usually commented-out, as in this JavaScript example:

<script type="text/javascript">
//<![CDATA[
document.write("<");
//]]>
</script>
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我想做一个坏孩纸
5楼-- · 2019-01-03 05:16

CDATA is Deprecated.

So do not use it in HTML 5.

https://developer.mozilla.org/en-US/docs/Web/API/CDATASection

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Animai°情兽
6楼-- · 2019-01-03 05:16

A way to write a common subset of HTML and XHTML, in the hope of greater portability.

In HTML, <script> is magic escapes everything until </script> appears.

So you can write:

<script>x = '<br/>';

and <br/> won't be considered a tag.

This is why strings such as:

x = '</scripts>'

must be escaped like:

x = '</scri' + 'pts>'

See: Why split the <script> tag when writing it with document.write()?

But XML (and thus XHTML, which is a "subset" of XML, unlike HTML), doesn't have that magic: <br/> would be seen as a tag.

<![CDATA[ is the XHTML way to say:

don't parse any tags until the next ]]>, consider it all a string

The // is added to make the CDATA work well in HTML as well.

In HTML <![CDATA[ is not magic, so it would be run by JavaScript. So // is used to comment it out.

The XHTML also sees the //, but will observe it as an empty comment line which is not a problem:

//

That said:

  • compliant browsers should recognize if the document is HTML or XHTML from the initial doctype <!DOCTYPE html> vs <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
  • compliant websites could rely on compliant browsers, and coordinate doctype with a single valid script syntax

But that violates the golden rule of the Internet:

don't trust third parties, or your product will break

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时光不老，我们不散
7楼-- · 2019-01-03 05:24

All text in an XML document will be parsed by the parser.

But text inside a CDATA section will be ignored by the parser.

CDATA - (Unparsed) Character Data

The term CDATA is used about text data that should not be parsed by the XML parser.

Characters like "<" and "&" are illegal in XML elements.

"<" will generate an error because the parser interprets it as the start of a new element.

"&" will generate an error because the parser interprets it as the start of an character entity.

Some text, like JavaScript code, contains a lot of "<" or "&" characters. To avoid errors script code can be defined as CDATA.

Everything inside a CDATA section is ignored by the parser.

A CDATA section starts with "<![CDATA[" and ends with "]]>"

Use of CDATA in program output

CDATA sections in XHTML documents are liable to be parsed differently by web browsers if they render the document as HTML, since HTML parsers do not recognise the CDATA start and end markers, nor do they recognise HTML entity references such as &lt; within <script> tags. This can cause rendering problems in web browsers and can lead to cross-site scripting vulnerabilities if used to display data from untrusted sources, since the two kinds of parsers will disagree on where the CDATA section ends.

A brief SGML tutorial.

Also, see the Wikipedia entry on CDATA.

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