It is impossible to use .attr('checked') to read and modify checkbox states for documents that have the application/xhtml+xml content type. Instead, it returns "checked" or "undefined" depending on whether the checked="checked" attribute is specified for the tag. A boolean true/false was expected.

If the document is sent as text/html, it works as expected.

I have tested it in both Firefox and Google Chrome with the same result.

Example: http://caffie.net/checkbox/index.xhtml