There are situations a component/control developer wants portion of the SG below itself to skipped by the CSS-Engine. The following reasons come to my mind:
* you have a very complex and deep scenegraph where parts are not shown to the user so processing those because of CSS-Changes might not make sense (eg detaching a TabPane with 100s of Tabs with Deep-Scenegraphs)
* you want to rule out side-effects produced by incorrect/to global CSS-Selectors and/or you want to make sure that what your controls displays is not hacked via CSS (eg display a PDF)

In Java8 there was the possibility to make impl_processCSS a no-op which is not possible anymore in Java9.

I would propose the following API
{code:java}
private boolean cssSensitive = true;

public boolean isCssSensitive() {
   return cssSensitive;
}

public void setCssSensitive(boolean cssSensitive) {
   if( this.cssSensitive != cssSensitive ) {
       this.cssSensitive = cssSensitive;
       if( cssSensitive ) {
           reapplyCSS(); // or is applyCSS() not sure
       }
   }
}
{code}

If I'd had a wish getting a package-scoped boolean field in JavaFX9 to make doProcessCSS() a no-op would provide me a way to carry over the performance improvements from Java8 to Java9