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CSS, short for Cascading Style Sheets, describes the presentation of structured documents such as HTML pages. A CSS selector identifies elements to which a rule applies; a CSS declaration assigns a property and value. The CSS cascade resolves competing declarations, while some properties can inherit values from an element's parent.^[ref]

For example, consider the rules p { color: blue; } and .warning { color: red; }. A paragraph with class="warning" matches both CSS selectors. For these ordinary rules in the same styling context, the class selector has greater CSS specificity than the element selector, so the paragraph is red even if the blue rule is written later. A CSS cascade therefore does more than choose whichever declaration was encountered last.^[ref]

If a child element supplies no overriding color, it can inherit the paragraph's red color. That does not mean every property is inherited: properties define their own inheritance behavior. A width declaration such as width: 200px also concerns layout in CSS pixels; it does not promise a width of 200 physical display pixels.^[ref]

Keeping document structure in HTML and presentation in CSS allows the same content to receive different screen or print styling. The rules describe the desired presentation; the browser determines the layout and rendered image from the document, styles and output device.

A CSS property is a defined parameter of presentation or layout, such as color, width or margin-left. Its definition specifies allowed values, an initial value, whether it inherits, and how values are computed. A CSS declaration proposes a value for a property on the elements matched by its rule.^[ref]

For example, width: 50% names the width property and supplies a percentage value. If that percentage resolves against a containing width of 400 CSS pixels, the resulting width is 200 CSS pixels in the simple case. The same declaration in a 600-pixel containing width can yield 300 pixels. A property name therefore identifies a parameter, while its eventual layout value can depend on context.

Different properties have different value grammars and defaulting behavior. A color word can be valid for color but invalid for width. Foreground color normally inherits, while width does not inherit by default. The absence of a width declaration is therefore different from automatically copying the parent's width.^[ref]

A property also differs from an HTML attribute with similar wording. Attribute interpretation is part of the element's document semantics; a CSS property participates in the styling value process, including the CSS cascade. Keeping the property definition, individual declarations and the resulting CSS used value separate makes styling behavior much easier to explain.