Selectors

Basic Selectors

*	Universal — all elements
div	Type — all elements
.class	Class — elements with class
#id	ID — element with id
[attr]	Attribute — has attribute
[attr="val"]	Attribute equals value

Combinators

A B	Descendant (any depth)
A > B	Direct child only
A + B	Adjacent sibling (next)
A ~ B	General sibling (all after)

Pseudo-classes

: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

::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

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

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

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

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

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

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

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

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

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

max-width: 480px	Mobile phones
max-width: 768px	Tablets
max-width: 1024px	Small laptops
max-width: 1280px	Desktops

Viewport Units

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

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

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); }

CSS QUICK REFERENCE