CSS · Cascading Style Sheets
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CSS is the language that styles a web page — the colors, fonts, spacing, and layout. If HTML is the skeleton, CSS is the skin and clothes: it takes that plain structure and decides how everything actually looks.
You write CSS as a list of rules. Each rule picks something on the page (a 'selector') and then sets its properties: make every heading dark blue, give paragraphs a comfy line height, lay the navigation out in a row. Change one rule and every matching element updates at once.
The 'cascading' part is the clever bit: when several rules touch the same element, CSS has a clear set of priorities for which one wins. That's how a site keeps a consistent look while still letting you override just one button or one page when you need to.
h1 {
color: navy;
font-size: 2rem;
}One rule: make every <h1> heading navy and a bit bigger.
HTML and CSS split work on purpose — structure in one place, looks in another. That's why you can totally restyle a site without touching its content.