<textarea name="test" pattern="[a-z]{1,30}(,[a-z]{1,30})*" id="test"></textarea>​

jQuery('#test').on('keyup',function() {
    jQuery(this).parent().append('<p>' + this.checkValidity() + ' ' +
    this.validity.patternMismatch + '</p>');
});

http://jsfiddle.net/Riesling/jbtRU/9/