css

Every element is a rectangular box composed of content, padding, border, and margin layers — and the box-sizing property determines whether width and height apply to just the content (content-box) or include padding and border (border-box).

Flexbox is a one-dimensional layout model for distributing space and aligning items along a main axis and cross axis, with container properties controlling flow direction and item properties controlling grow, shrink, and basis behavior.

The five position values (static, relative, absolute, fixed, sticky) each change how an element relates to the document flow and its containing block, while z-index and stacking contexts control layering order for overlapping elements.

Responsive design combines the viewport meta tag, media queries, fluid units (%, vw, rem, clamp()), and mobile-first methodology to create layouts that adapt gracefully from mobile screens to large desktops.

CSS selectors target elements for styling using type, class, ID, attribute, and pseudo-class/element patterns, with specificity calculated as a weighted score (inline > ID > class > element) that determines which rule wins when multiple selectors match the same element.