Because the spec uses HTML to normatively define how Markdown should be rendered, it should cite the HTML5 specification (and/or the other HTML5 specification… heh) as the reference for how the HTML used in the standard should be interpreted and rendered. I think this is particularly important since it is a deviation from Markdown.pl, and HTML5 elements referred to explicitly in a few places in the specification.
As a random example, I like the way the [OpenCL C specification] (https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf) defines its relationship to ISO C99.