I tried to integrate yui4jsf into an existing project which uses Facelets and outputs strict XHTML. The artifacts output by yui4jsf were not valid XML; in particular, the ampersands in the URLs for the scripts and stylesheets were not escaped. Also I noticed that the JavaScript was not escaped as well or wrapped in CDATA. This causes the pages to be rejected by Firefox, although IE 6 ignored the errors. Any page which was access via the filter was affected.
I was using version 0.6.1.