Because it default clones whatwg/html, if I directed a first-time contributor here, the instructions in the readme would get them stuck, unable to push after making changes.
I'd suggest a prompt in build.sh that asks them where to check out from. I am thinking:
Didn't find the HTML source on your system...
Enter where you would like to clone it from (GitHub username or URL):
(URL detection can be done by looking for :.)
Alternately a quick fix is just to update the readme to suggest checking out your fork in a sibling directory before building for the first time.