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.
To preview your changes locally, follow the instructions in the html-build repository.
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.
Please follow the guidelines for writing good commit messages.
The HTML Standard is complex and people notice minor (and large) issues with it all the time. We'd love your help fixing these.
We've collected a list of good first bugs that you could help us fix, to get a taste for how to submit pull requests, how the build process works, and so on:
We'd be happy to mentor you through this process. If you're interested and need help getting started, leave a comment on the bug or issue, or ask around in IRC.