If you wish to build a website which is compatible with IE 6 (This is end month of 2013 where it has ie 11) I recommend using YAML http://www.yaml.de/ . It has clear documentation with examples. You can also use http://www.cascade-framework.com/ which has minimum CSS which required for a website/web application development. Also, YUI you can use http://yuilibrary.com/yui/environments/ .
Hope it helps.