This repository hosts the HTML Standard.
The short version is that all you need to do is change the source resource and submit a patch. The longer version is that you probably want to read the WHATWG FAQ first.
The source for the spec is formatted in such a way that its paragraphs have line breaks after about every 100 characters or so. Any patch you contribute must follow that same formatting convention. So, use whatever option your text editor may provide for automatically doing that kind of "hard" wrapping of lines at 100 characters; for example, in Emacs, set fill-column to 100; in Vim, set textwidth to 100; and in Sublime, set wrap_width to 100.
The green button shall not be pushed. Each change needs to result in a single commit on the master branch.