So much depends on how we define science fiction and fantasy, heh, and since both are speculative fiction, the line between speculation and pure fantasy may become very thin indeed for science fiction.
A thin line indeed, and one that I think earlier generations didn't worry themselves about as much as some readers/writers do today. Is Pern SF or fantasy? Does it matter? Just write the story that needs to be written with the elements necessary to tell the story. Got faeries? Cool. Got skyships? Cool. Got faeries that pilot skyships? Cool. No matter what the intentions of the author are, readers may or may not receive the work if they have preconceived notions about what constitutes "fantasy" and "science fiction." Their loss. I never considered my WIP trilogy fantasy per se, but I'm told my style, worldbuiling, character-driven plot, and lack of "space/tech stuff" will make marketing it as fantasy a better choice. Okay, I guess. I guess I had better get used to those reviews that I see on other people's second world/soft SF/science fantasy books: "this really isn't fantasy because there's no magic system/Elder Races/dragons," etc. Yawn.