While you may have already been using CSS 3 in your ebooks, EPUB 3 has not officially supported it. Sure, it worked but it wasn’t required to. So what you were doing was actually a progressive enhancement.
BookNet Canada is a non-profit organization that develops technology, standards, and education to serve the Canadian book industry. Founded in 2002 to address systemic challenges in the industry, BookNet Canada supports publishing companies, booksellers, wholesalers, distributors, sales agents, industry associations, literary agents, media, and libraries across the country.