15px

15px is a length value used in styling web pages. In CSS, 15px denotes fifteen CSS pixels, a unit defined by the CSS specification. CSS pixels are an abstract measure that maps to device pixels according to the display’s pixel density; on standard-density screens, a CSS pixel is roughly one device pixel, while on high-density screens a CSS pixel can correspond to multiple device pixels. This abstraction helps maintain consistent sizing across devices with different screen resolutions.

The 15px unit is commonly applied to font sizes, margins, padding, borders, and image dimensions. For example, a declaration such as font-size: 15px; or padding: 15px; sets those properties to a fixed, absolute size within the layout. The exact rendering depends on the context, including the browser’s zoom level and the device’s display characteristics.

Because px is an absolute unit, its size does not automatically scale with the user’s base font size or accessibility settings. For scalable typography and layouts, many designers prefer relative units such as rem (root em) or em, possibly in combination with media queries to adapt to different viewports. Using relative units can improve readability for users who adjust font sizes for accessibility.

Cross-browser support for px values is universal, and 15px is widely accepted in modern web design. When designing with 15px, developers should consider responsive behavior, accessibility implications, and how the chosen values interact with other units in a given layout. See also CSS length units, pixel, rem, em, and responsive design.