For index.html hosting, the web server uses index.html as the default homepage file. When someone opens your domain, it looks for that file at the root of the deployed website.
After a ZIP upload, a 404 often means the archive contains project/index.html instead of index.html. Extract the ZIP, enter the inner project folder, select the files inside it, and create a new archive.
Name the file exactly index.html in lowercase. Then check that its CSS and image paths are relative to the same root. Publish the corrected ZIP and verify the hosted URL in a private browser window.