CSS (Cascading Style Sheets) controls visual presentation of HTML — colours, fonts, layouts, spacing, and animations. CSS rules consist of a selector (what to style) and declarations (how to style it).
Why does it matter?
Without CSS, web pages are plain text. Every design element — colours, fonts, spacing, layouts — comes from CSS. Understanding the box model and cascade prevents the most common layout bugs that frustrate beginners.
Learn CSS fundamentals — selectors, specificity, the box model (margin, border, padding, content), display types, and the cascade.
Real-World Use Cases
🎨Product card styling - Padding inside the card, border around it, margin between cards — the box model controls every spacing decision.
📱Responsive layout - CSS display properties and the box model are the foundation of every responsive grid and flexbox layout.
🖼️Button design - Border-radius rounds corners, padding adds internal spacing, background-color sets the fill — pure CSS.
🗂️Navigation bar - Horizontal nav uses display:flex, with padding on links and a border-bottom separating nav from content.
CSS Selectors and Specificity
p { color: #374151; } /* element — lowest specificity */
.btn { background: #6366f1; } /* class */
#header { height: 60px; } /* ID — high specificity */
nav a { text-decoration: none; } /* descendant */
nav > a { font-weight: 600; } /* direct child */
a:hover { color: #4f46e5; } /* pseudo-class */
li:first-child { font-weight: bold; }
input:focus { outline: 2px solid #6366f1; }
p::first-line { font-size: 1.2em; } /* pseudo-element */
li::before { content: "v "; color: #6366f1; }
input[type="email"] { border-color: #10b981; } /* attribute */
The Box Model
/* Every element: Content, Padding, Border, Margin */
.card {
width: 300px;
padding: 20px; /* space INSIDE the border */
border: 1px solid #e2e8f0;
border-radius: 8px;
margin-bottom: 16px; /* space OUTSIDE the border */
}
/* ALWAYS add this to every project */
* { box-sizing: border-box; }
/* Makes width INCLUDE padding and border (intuitive!) */
Display Types
/* block: full width, new line (div, p, h1) */
.block { display: block; }
/* inline: stays in text flow, no width/height (span, a) */
.inline { display: inline; }
/* inline-block: inline flow + accepts width/height */
.badge { display: inline-block; padding: 2px 8px; }
/* none: completely removed */
.hidden { display: none; }
/* flex and grid — covered in dedicated posts */
.nav { display: flex; gap: 16px; }
.grid { display: grid; grid-template-columns: repeat(3, 1fr); }
Q: What is the difference between margin and padding?
Padding is the space INSIDE the element between content and border — it has the element background color. Margin is space OUTSIDE the element between its border and neighboring elements — always transparent. Use padding for internal spacing; use margin for spacing between elements.
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