CSS basics and how browsers apply it

Purpose of CSS for web page styling

CSS (Cascading Style Sheets) is used to control how HTML content looks when a browser renders it. While HTML structures a document, CSS determines the styling—such as text appearance, colors, spacing, and overall layout—so pages can be designed beyond the browser’s basic default styles. Browsers apply CSS by first converting the HTML into a DOM tree, then collecting CSS rules (from inline styles or external .css files), matching them to elements using selectors, and producing a render tree that is finally painted to the screen. CSS is rule-based: selectors target elements, and declaration blocks assign property/value pairs (e.g., setting heading color and font size).

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Browser default styles for HTML readability

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