# Initial containing block

The top-level box, the size of the viewport.

## What is the **initial containing block**?

The **initial containing block** is the top-level box, the size of the viewport, that everything else ultimately sits in.

```css
html { height: 100%; }   /* 100% of the initial containing block */
```

Level: Master · [More CSS terms at this level](https://www.ask-codey.com/glossary/subjects/css/master)

Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/initial-containing-block