The HTMLifier "converts" Scratch 3.0 projects to an HTML file by putting all the project data and the entire Scratch engine into one enormous file.
It does this by making Scratch VM fetch a project, and in doing so, it tracks the assets it fetches from the project.json. It converts the fetched assets and project.json into a base64 data URI. It also fetches the code for the Scratch VM. It inserts all this in a template HTML file, which has been set up to load the project from the base64 data URIs and do other things that the Scratch VM doesn't take of, which is normally handled by scratch-gui, such as variable/list monitors and ask and wait prompts.
hacky-file-getter.js is based on Scratch VM's benchmark page.
CSS by Mr. Cringe Kid.
The Scratch engine is from scratch-vm.
download.js is used to download the HTML file.