Out of the box, Safari displays your existing websites beautifully on the edge-to-edge display of the new iPhone X. Content is automatically inset within the display’s safe area so it is not obscured by the rounded corners or the device’s sensor housing.
The inset area is filled with the page’s background-color (as specified on the or elements) to blend in with the rest of the page. For many websites, this is enough. If your page has only text and images above a solid background color, the default insets will look great.
Source: Designing Websites for iPhone X