AsciiDoctor is a second implementation that doesn't even fully compatibly implement the original specification. The original AsciiDoc is pretty well-specified, and it's mostly the plaintext markup of stuff that was intended to go to DocBook, with very little surprises from that.
AsciiDoctor pretty much focused on a direct HTML translation and ignored the inconvenient parts. (Some of the inconvenient parts are deprecated syntax that while AsciiDoc's had a replacement for, I've written the old style for ~20 years and when GitHub tries to render a document with AsciiDoctor, oops; sometimes I'll change the document, sometimes I'll decide rendering on GitHub isn't important.)
AsciiDoctor pretty much focused on a direct HTML translation and ignored the inconvenient parts. (Some of the inconvenient parts are deprecated syntax that while AsciiDoc's had a replacement for, I've written the old style for ~20 years and when GitHub tries to render a document with AsciiDoctor, oops; sometimes I'll change the document, sometimes I'll decide rendering on GitHub isn't important.)