# CSS Quick Reference

*Selectors, layout, flexbox, grid, animation, responsive design*

> Source: MDN Web Docs (developer.mozilla.org) · MIT

## Selectors

### Basic Selectors

| Command | Description |
|---------|-------------|
| `*` | Universal — all elements |
| `div` | Type — all `<div>` elements |
| `.class` | Class — elements with class |
| `#id` | ID — element with id |
| `[attr]` | Attribute — has attribute |
| `[attr="val"]` | Attribute equals value |

### Combinators

| Command | Description |
|---------|-------------|
| `A B` | Descendant (any depth) |
| `A > B` | Direct child only |
| `A + B` | Adjacent sibling (next) |
| `A ~ B` | General sibling (all after) |

### Pseudo-classes

| Command | Description |
|---------|-------------|
| `:hover` | Mouse over element |
| `:focus` | Element has focus |
| `:first-child` | First child of parent |
| `:nth-child(n)` | Nth child (1-based, `odd`, `even`, `2n+1`) |
| `:not(sel)` | Negation — exclude matches |
| `:has(sel)` | Parent selector (contains match) |

### Pseudo-elements

| Command | Description |
|---------|-------------|
| `::before` | Insert content before element |
| `::after` | Insert content after element |
| `::placeholder` | Style input placeholder text |
| `::selection` | Style highlighted text |

## Box Model

### Box Sizing

```
/* Include padding/border in width */
*, *::before, *::after {
  box-sizing: border-box;
}
```

### Properties

| Command | Description |
|---------|-------------|
| `margin` | Space outside border |
| `border` | Edge around padding |
| `padding` | Space inside border |
| `width / height` | Content dimensions |
| `outline` | Ring outside margin (no space) |

### Shorthand

```
margin: 10px;           /* all sides */
margin: 10px 20px;      /* vertical | horizontal */
margin: 10px 20px 30px; /* top | horiz | bottom */
margin: 10px 20px 30px 40px; /* T R B L */
```

## Flexbox

### Container

```
.flex {
  display: flex;
  justify-content: center; /* main axis */
  align-items: center;     /* cross axis */
  gap: 1rem;
}
```

### Container Properties

| Command | Description |
|---------|-------------|
| `flex-direction` | `row` \| `column` \| `row-reverse` \| `column-reverse` |
| `flex-wrap` | `nowrap` \| `wrap` \| `wrap-reverse` |
| `justify-content` | `flex-start` \| `center` \| `space-between` \| `space-around` \| `space-evenly` |
| `align-items` | `stretch` \| `center` \| `flex-start` \| `flex-end` \| `baseline` |
| `align-content` | Multi-line cross axis alignment |
| `gap` | Space between flex items |

### Item Properties

| Command | Description |
|---------|-------------|
| `flex: 1` | Grow to fill available space |
| `flex: 0 0 200px` | Fixed width, no grow/shrink |
| `align-self` | Override align-items for one item |
| `order` | Change visual order (default 0) |

## Grid

### Container

```
.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: auto 1fr auto;
  gap: 1rem;
}
```

### Container Properties

| Command | Description |
|---------|-------------|
| `grid-template-columns` | Define column tracks: `1fr 2fr`, `repeat(3, 1fr)` |
| `grid-template-rows` | Define row tracks |
| `grid-template-areas` | Named areas: `"header header" "nav main"` |
| `gap` | Row and column gaps |
| `justify-items` | Align items inline (horizontal) |
| `align-items` | Align items block (vertical) |

### Item Placement

```
.item {
  grid-column: 1 / 3;   /* span cols 1-2 */
  grid-row: 1 / -1;     /* span all rows */
  grid-area: header;    /* named area */
}
```

## Typography

### Font Properties

| Command | Description |
|---------|-------------|
| `font-family` | Typeface stack: `'Inter', sans-serif` |
| `font-size` | Size: `1rem`, `16px`, `clamp(1rem, 2vw, 2rem)` |
| `font-weight` | `normal` (400) \| `bold` (700) \| `100`-`900` |
| `font-style` | `normal` \| `italic` \| `oblique` |
| `line-height` | Line spacing: `1.5` (unitless recommended) |
| `letter-spacing` | Character spacing: `0.05em` |

### Text Properties

| Command | Description |
|---------|-------------|
| `text-align` | `left` \| `center` \| `right` \| `justify` |
| `text-decoration` | `none` \| `underline` \| `line-through` |
| `text-transform` | `uppercase` \| `lowercase` \| `capitalize` |
| `text-overflow` | `ellipsis` (with overflow: hidden) |
| `white-space` | `nowrap` \| `pre` \| `pre-wrap` |
| `word-break` | `break-all` \| `break-word` |

## Colors & Backgrounds

### Color Formats

```
color: #ff6600;             /* hex */
color: rgb(255, 102, 0);    /* rgb */
color: hsl(24, 100%, 50%);  /* hsl */
color: oklch(70% 0.15 50);  /* oklch */
```

### Background Properties

| Command | Description |
|---------|-------------|
| `background-color` | Solid fill: `#f0f0f0` |
| `background-image` | `url(img.jpg)` or gradient |
| `background-size` | `cover` \| `contain` \| `100px 200px` |
| `background-position` | `center` \| `top right` \| `50% 50%` |
| `background-repeat` | `no-repeat` \| `repeat-x` \| `repeat-y` |

### Gradients

```
background: linear-gradient(to right, #f00, #00f);
background: radial-gradient(circle, #fff, #000);
background: conic-gradient(red, yellow, green);
```

## Transitions & Animation

### Transitions

```
.btn {
  transition: background 0.3s ease, transform 0.2s;
}
.btn:hover {
  background: #0056b3;
  transform: scale(1.05);
}
```

### Transition Properties

| Command | Description |
|---------|-------------|
| `transition-property` | Which property to animate |
| `transition-duration` | Length: `0.3s`, `300ms` |
| `transition-timing-function` | `ease` \| `linear` \| `ease-in-out` \| `cubic-bezier()` |
| `transition-delay` | Wait before starting |

### Keyframe Animation

```
@keyframes spin {
  from { transform: rotate(0deg); }
  to   { transform: rotate(360deg); }
}
.icon { animation: spin 1s linear infinite; }
```

### Animation Properties

| Command | Description |
|---------|-------------|
| `animation-name` | Reference @keyframes name |
| `animation-duration` | Length of one cycle |
| `animation-iteration-count` | `1` \| `3` \| `infinite` |
| `animation-direction` | `normal` \| `alternate` \| `reverse` |
| `animation-fill-mode` | `forwards` \| `backwards` \| `both` |

## Responsive Design

### Media Queries

```
@media (max-width: 768px) {
  .sidebar { display: none; }
}
@media (prefers-color-scheme: dark) {
  body { background: #1a1a1a; color: #eee; }
}
```

### Common Breakpoints

| Command | Description |
|---------|-------------|
| `max-width: 480px` | Mobile phones |
| `max-width: 768px` | Tablets |
| `max-width: 1024px` | Small laptops |
| `max-width: 1280px` | Desktops |

### Viewport Units

| Command | Description |
|---------|-------------|
| `vw / vh` | % of viewport width / height |
| `dvh` | Dynamic viewport height (mobile-safe) |
| `svh / lvh` | Small / large viewport height |
| `cqi` | Container query inline size |

### Container Queries

```
.card-wrapper { container-type: inline-size; }
@container (min-width: 400px) {
  .card { flex-direction: row; }
}
```

## Positioning

### Position Values

| Command | Description |
|---------|-------------|
| `static` | Default — normal document flow |
| `relative` | Offset from normal position; keeps space |
| `absolute` | Positioned to nearest positioned ancestor |
| `fixed` | Positioned to viewport; stays on scroll |
| `sticky` | Toggles relative/fixed based on scroll |

### Centering Patterns

```
/* Flex center */
display: flex;
justify-content: center;
align-items: center;

/* Grid center */
display: grid;
place-items: center;
```

### Stacking

| Command | Description |
|---------|-------------|
| `z-index` | Stack order (higher = on top); needs `position` |
| `isolation: isolate` | Creates new stacking context |

## Custom Properties

### Define & Use

```
:root {
  --color-primary: #3b82f6;
  --spacing-md: 1rem;
}
.btn {
  background: var(--color-primary);
  padding: var(--spacing-md);
}
```

### Fallback Values

```
color: var(--accent, #ff6600);
/* uses #ff6600 if --accent is not defined */
```

### Dynamic Theming

```
[data-theme="dark"] {
  --bg: #1a1a2e;
  --text: #e0e0e0;
}
body { background: var(--bg); color: var(--text); }
```