During the course of JSF 1.0 development, there was a time when we had a strict
TLV in the Sun JSF RI that enforced the validity of your JSF jsp pages. This
caused big problems for tools because, if you're using a real runtime to render
your JSP page as you author it in an iterative fashion, there will be times when
the page isn't valid (simply because the user hasn't yet gotten around to
filling in all the values) but you still want it to render for the benefit of
the user experience. The tools vendors convinced the Sun JSF RI team to make
their TLVs check for java.beans.beans.Beans.isDesignTime() == true, and if so,
disable the TLV from throwing exceptions.
I think this is a good "best practice" regarding JSP toolability.