To "compile" the specification in HTML format, you must first install the Sphinx documentation generator.
If you're running Ubuntu Linux:
sudo apt install make python3-pip pip3 install -U pip pip3 install -r requirements.txt
If you're running macOS or another operating system, see the instructions at https://www.sphinx-doc.org/en/master/usage/installation.html
Just type:
make html
And a set of HTML pages will be generated inside _build/html.
You can also type:
make latexpdf
Sphinx will generate a .tex file and will try to create a .pdf document from it using pdflatex (which must be installed).
For further options, type make help.