I'm glad the article mentioned no flip-flops. I've seen women wear them to otherwise semi-formal events and they looked tacky.
I went to an afternoon wedding a couple of months ago (summer wedding) and my skirt was below my knees, I wore light colored pantyhose. The bride was the only other woman at the event wearing hose. I really couldn't believe it. Personally, I think pantyhose should always be considered a necessary garment when attending a semi-formal event. Men always wear socks, and women should wear hose to such events.
Fine jewelry and designer labels are not necessary to wear to a semi-formal event, but pantyhose should be required.