I tried running it through a MultiMarkdown convertor but got too many errors.
You can use pandoc to create it, as it has a commonmark reader and an epub writer:
pandoc -f commonmark -o spec.epub spec.txt
spec.txt isn’t a Markdown file – it uses a few special conventions. So this command won’t work. You’d have better luck using pandoc to convert the HTML version of the spec to epub. But to make it look good, you’ll also have to add some custom CSS.