Pixels (px) are a fixed unit of measurement used in web design. They are based on the resolution of the display, meaning their size remains consistent across different devices and screen sizes. For example, an element set to 100px wide will always be 100 pixels wide, whether it’s viewed on a mobile phone or a large monitor. This makes px a great choice for elements that require precise dimensions, such as images, icons, or fixed layouts. However, since it doesn’t adapt to changes in the user’s browser settings or screen resolution, using pixels can lead to a lack of responsiveness and accessibility in some designs.
EM is a relative unit of measurement that scales based on the font size of the root HTML element (). By default, most browsers set this root size to 16px, so 1em typically equals 16px. What makes em powerful is its ability to create scalable and responsive layouts. When a user changes their browser’s default font size or zoom level, elements using em automatically adjust, ensuring better accessibility and a more flexible design. This makes em ideal for font sizes, padding, margins, and other components that need to respond to various screen sizes and user preferences.