90vw

90vw is a CSS length value that uses the viewport width unit. The vw unit represents 1% of the width of the viewport, so 90vw equals 90% of the visible width of the browser window or device screen. It is relative to the viewport itself, not to the width of a parent element, which makes it useful for responsive, near-full-width layouts.

Common uses include setting a container’s width to 90vw for layouts that nearly fill the screen, creating hero sections, or sizing modal backdrops that should cover most of the screen while leaving a margin for UI controls on smaller devices. Because vw units track the viewport, elements sized with 90vw resize automatically when the window is resized or when the device orientation changes.

Examples of consideration and behavior: on a 1366px-wide display, 90vw is approximately 1229px; on a smaller device, the value scales down accordingly. However, 90vw does not account for margins, padding, borders, or scrollbars in all contexts, which can lead to slight overflow or horizontal scrolling if combined with other wide elements or default body margins. To improve predictability, many designers reset body margins to zero and/or use additional constraints such as min-width, max-width, or the clamp function (for example, to keep a readable line length on very wide screens).

Browser support for viewport units is extensive across modern browsers. 90vw is a standard tool in responsive design for controlling width relative to the user’s viewport.