Are HTML attribute names really case-sensitive? A topic filled with conflicting information

After learning that attribute names are case-sensitive, I found conflicting information.

A source mentioned that attribute names are indeed case-sensitive. For example, the width attributes in <div> and <DIV> would be considered separate due to their different cases;

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Curious about this, I conducted an experiment on JSFiddle:

HTML

<p ID="myId">Let's see if this is red</p>

CSS

#myId {color: red;}

To my surprise, the text did turn red as expected: http://jsfiddle.net/xtLr08u7/.

This made me question which information is correct.

Answer №1

According to the W3C HTML Reference, attribute names are not case sensitive in both HTML 4 and HTML5.

However, it is important to note that XHTML (HTML with XML syntax) is case sensitive.

Answer №2

Custom attributes are unique identifiers that must be case-sensitive, unlike normal attributes which are not.

For example, in XML:

<tag myAttr="myattr" MyAttr="MYATTR"></tag>

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