I recently made plans to attend this year's South by Southwest conference and was reminded of a conversation I had at last year's SxSW. I was talking about RSS with someone who has long been a supporter of CSS-based web design, and he feared that the success of RSS meant that design has lost the battle to content. After all, RSS is all about content - when you read a site's feed in an aggregator like FeedDemon, you're not seeing the hard work put into that site's design. This lead to a comment that TopStyle and FeedDemon were in fact at odds with each other.
I hadn't considered that before, but it made sense. TopStyle is all about designing standards-compliant CSS-based designs, whereas FeedDemon enables skipping the design and just reading a site's content. But then it occurred to me that instead of being polar opposites, my programs are actually complementary. TopStyle's CSS creation enables the separation of layout from content, leading to smaller, faster-loading sites whose design information is contained in style sheets rather than interwoven with every page. This makes it much easier to repurpose a site's content for use in an RSS feed.
Plus, in many ways RSS is an offspring of blogging, and blogging tools rely heavily on CSS-based design. Just look at sites hosted by TypePad and Blogger, or sites which rely on blogging tools such as MovableType and WordPress - almost all of them use CSS to separate their layout from their content.
So, rather than being rivals, I think CSS has helped enable the spread of RSS.