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Note: This article was written before boolean arttributes were allowed - especially when writing XHTML.

I was recently sent this question:

I tried to use “checked” in an XHTML page. When I validated the page, this showed up as invalid. Why?

Answer:

The use of “checked” in this way is referred to as “attribute minimization”. This means the author has used the attribute’s name but has not specifying a value.

<input name="name" type="checkbox" checked />

For a document to be valid XHTML, attributes cannot be minimized. The W3C XHTML 1.0 spec states:

XML does not support attribute minimization. Attribute-value pairs must be written in full. Attribute names such as compact and checked cannot occur in elements without their value being specified.

http://www.w3.org/TR/xhtml1/#h-4.5

So, the correct way to write this code would be:

<input name="name" type="checkbox" checked="checked" />