Affixing an Element to the Browser Window
When a visitor scrolls in the browser window, the contents of the window usually move up or down while the Back and Forward buttons, for example, stay stationary or fixed. CSS allows you to affix elements to the browser window so that they don't move when the visitor scrolls up or down.
To affix an element to the browser window:
Figure 11.34. A fixed element lets other elements flow right over it. In addition, it doesn't move when the visitor scrolls through the page.
Figure 11.35. Notice how the sidebar remains immobile as the visitor scrolls through the left content.