Speculative fiction is usually defined as science fiction, fantasy and horror plus some odds and ends. scifi and fantasy are on-topic for this site, but horror is not. So would that odds and ends present in speculative fiction be on-topic here?
Earlier¹, I proposed to define the topic of this site as speculative fiction, with the ISFDB definition as a starting point. So that includes alternate history and, yes, horror. If “what if?” is a key element of the work, it's on-topic.
Hmm, we should work the words “speculative fiction” into the FAQ. We don't use it as the name of the site because the term is relatively little-known.
¹ Context note: when this site started, it was for science fiction only. This meta question was one of the steps on the road to broadening the topic.