If the skinning file has a skinning key with a pseudo-class that is not a CSS-2 pseudo-class (e.g., :read-only, :required), then it gets converted to a styleclass that starts with ".p_AF" (e.g., .p_AFReadOnly, .p_AFRequired)and it does not get put in the compression style map. This is exactly what we want because client-side code can add/remove the 'state' styleclass when the state changes, and if it was compressed they would have to send all states to with their compression equivalent to the client, which adds content.
This all works fine. If someone creates a skin css file and uses .p_AFReadOnly instead of :read-only, then ".p_AFReadOnly" gets put into the compressed styleclass map and it gets compressed when it is written to the html, and the clients add/remove code no longer works.
Add some sort of protection against this like a warning message at least.