CSS Units (em, rem)
Em and rem are relative CSS length units used for sizing elements in web development, where 'em' is relative to the font-size of the parent element and 'rem' (root em) is relative to the font-size of the root element (typically the <html> tag). They enable scalable and responsive designs by allowing sizes to adjust based on user preferences or viewport dimensions, improving accessibility and maintainability compared to fixed units like pixels.
Developers should use em and rem units when building responsive websites that need to adapt to different screen sizes or user font-size settings, as they provide flexibility and consistency across devices. Em is ideal for component-level styling where sizes should scale with local context, while rem is preferred for global sizing to maintain a consistent scale throughout the document, reducing complexity in CSS.