Section 35.4 Styling, and the First Cell

The first cell of every notebook is a collapsed code cell holding a small amount of styling: borders on theorem-like blocks, authored table rules, side-by-side layout, and similar light touches. The styling is entirely self-contained—it makes no network requests, so reading a notebook is a private activity, and everything works offline.

Jupyter’s security model decides when that styling takes effect. In a trusted notebook (one the reader built, or has executed, or has explicitly trusted) the styling applies the moment the notebook opens. In an unfamiliar notebook the reader runs the first cell once, and an instruction cell says exactly that. Either way, every notebook is fully readable with no styling at all—it is an enhancement, never a requirement. Note that some hosted environments (Google Colab, notebook previews on websites) isolate or ignore such styling entirely; content is unaffected.