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The matching criterion can be specified by using class
and id (=
identification) attributes and corresponding
selectors: class selector or id selector. According to the CSS2 specification the names inside
id attributes work as identifiers to elements.
The class selector is designed to many elements in the same document. The basic idea of using class selectors is to give the possibility to avoid the limitation of element type identifiers (element names). Using class selectors you can create as many "new elements" as you need.
Class and id selectors (like attribute What are selectors, classes and id-attributes, which I handle later) have higher weight as bare element type selectors. According to the HTML specification the name of the
id attribute should be document-wide unique. The
id attribute has been called also as the fragment
identifier. In principle an
id-attribute works as as an unique identifier of some fragment of a web page.
It is however remarkable that CSS-capable browsers accept in CSS the same value of the
id-attribute in several places in a document. In practise id selectors could be used (against HTML-specifications) in several places in the same document together with class selectors. In this case classes would define common properties and an id selectors would specify exceptional properties. Indeed the problem is that if documents are intended to check with some validator services, they complain errors. The same result can however be achieved by using for the same element several classes (I give an example further).
You can give quite freely names to class and id-selectors, but there is however some limitations:
_) has been afterward added into CSS2.
\(backslash) before them like
w\?. In theory it is also possible to use ISO 10646 characters 161 and above as a numeric code. It is possible to use character references inside style attributes (like
style="voice-family: Dü"), but these don't work in external files or inside the element
STYLE(they need escaped characters with backslashes).
Browsers accept illegal names. MS IE 5.x for Windows accepts them most. This matter has been fixed in MS IE 6.0 for Windows in the standard-compliant mode. Also most Netscape and Opera browsers accept illegal names but less than MS IE 5.x for Windows. I have a a model pagein order to test this matter.
Some browsers handle always the values of the
class as case-sensitive, but some browsers don't. Use is rules, which refer
id attributes in name the same cases, which are in the actual documents.
I don't recommend to use any special character, because according my tests they work (at least partial) in Netscape 6.x+ and MS IE 6.0 Windows browsers.