Simon Willison forked gistpreview.github.io to create gisthost.github.io, a tool that renders HTML files stored in GitHub Gists. The original gistpreview uses GitHub Pages and the Gist API to fetch and display HTML content via document.write(), bypassing GitHub's text/plain content-type restriction. The new fork fixes two issues: Substack URL mangling and truncated large files from the API. It also removes jQuery and Bootstrap dependencies, consolidating everything into a single HTML