Doing a complete difference of the showcase application running under Mojarra 2.1.21 versus 2.2.0 indicates that the only likely difference is the DOCTYPE.
With Mojarra 2.1.21, the showcase application has the following DOCTYPE:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
And with Mojarra 2.2.0, it is:
<!DOCTYPE html>
The actual relevant .xhtml file(s) use the xhtml transitional DOCTYPE, so it's not clear why that's being suppressed with Mjoarra 2.2.0.
Aftering seeing this article, I tested adding the complete DOCTYPE to the composition client(s) as well, but tht did not help.
http://stackoverflow.com/questions/15257832/mojarra-2-2-snapshot-and-doctypes
Another relevant article:
https://community.jboss.org/thread/223261?_sscc=t
When reproducing the issue in showcase, by running with Mojarra javax.faces-2.2.0-m15 versus Mojarra 2.1.21, the document head markups are the same, the coalesced CSS files are the same, and the actual clickable tab markups are the same.