Custom data in XHTML 1.0 Strict

You can use the jQuery MetaData plugin which allows you to write data in the class attribute in JSON format:

<li class="someclass {some: 'data'} anotherclass">...</li>

Then in your jQuery, to get at the data:

var data = $('li.someclass').metadata();
if ( data.some && data.some == 'data' )
    alert('It Worked!');

This should meet your requirements of being xhtml 1.0 strict, and also allows you to use a plug-and-play solution :)