The Complete CSS Guide

/* selector { property: value; } */
h1 { color: navy; font-size: 2rem; }

p            /* element */
.card        /* class — reusable, your main tool */
#header      /* id — unique, avoid for styling */
*            /* universal */
h1, h2       /* group */

div p        /* descendant — any depth */
div > p      /* direct child only */
h2 + p       /* adjacent sibling — the p right after h2 */
h2 ~ p       /* all following siblings */

[type="email"]      /* attribute equals */
[href^="https"]     /* starts with */
[href$=".pdf"]      /* ends with */
[class*="btn"]      /* contains */

li:first-child      li:last-child      li:nth-child(2n)  /* even */
li:nth-child(odd)   p:not(.intro)     input:checked
div:has(> img)      /* parent selector! — modern browsers */
li:is(.a, .b)       /* matches either, shorter to write */

color: #6d6ffb;                    /* hex */
color: rgb(109 111 251);           /* rgb — modern space syntax */
color: rgb(109 111 251 / 0.5);     /* with 50% alpha */
color: hsl(239 95% 71%);           /* hue saturation lightness — easiest to tweak */

background: linear-gradient(135deg, #6d6ffb, #8b5cf6);
background: radial-gradient(circle at top, #1c2333, #0a0a10);
background: conic-gradient(from 0deg, red, yellow, red);  /* pie charts! */

/* every element = content + padding + border + margin */
.box {
  width: 300px;
  padding: 16px;        /* inside the border */
  border: 2px solid;    /* the edge */
  margin: 24px;         /* outside — pushes neighbours away */
}

/* THE most important rule in CSS — put it in every project: */
*, *::before, *::after { box-sizing: border-box; }
/* now width:300px INCLUDES padding and border */

margin: 10px 20px;         /* vertical | horizontal */
margin: 5px 10px 15px 20px;/* top right bottom left (clockwise) */
margin-inline: auto;       /* center a block element */

body {
  font-family: 'Inter', system-ui, sans-serif;  /* always give fallbacks */
  font-size: 1rem;
  font-weight: 400;        /* 100–900; 400 normal, 700 bold */
  line-height: 1.7;        /* unitless = multiplier — best practice */
  letter-spacing: -0.01em;
}
h1 { text-transform: uppercase; text-align: center; }
a  { text-decoration: underline; text-underline-offset: 3px; }
p  { text-overflow: ellipsis; white-space: nowrap; overflow: hidden; } /* truncate */

/* multi-line clamp */
.excerpt { display: -webkit-box; -webkit-line-clamp: 3;
           -webkit-box-orient: vertical; overflow: hidden; }

position: static;    /* default — in normal flow */
position: relative;  /* nudged from its spot; becomes anchor for children */
position: absolute;  /* removed from flow; positioned vs nearest non-static parent */
position: fixed;     /* pinned to the viewport — navbars, chat buttons */
position: sticky;    /* scrolls until it hits top: 0, then sticks */

.tooltip {
  position: absolute;
  top: 100%; left: 50%;
  transform: translateX(-50%);
  z-index: 10;         /* stacking order — needs a position set */
}

.container {
  display: flex;
  flex-direction: row;          /* row | column */
  justify-content: space-between; /* main axis: start|center|end|space-between|around|evenly */
  align-items: center;          /* cross axis: stretch|start|center|end|baseline */
  flex-wrap: wrap;
  gap: 16px;                    /* spacing between items — use this, not margins */
}
.item {
  flex: 1;              /* grow to share space equally */
  flex: 0 0 200px;      /* grow shrink basis — fixed 200px */
  align-self: flex-end; /* override for one item */
  order: -1;            /* visual reorder */
}

/* the centering everyone googles: */
.center { display: flex; justify-content: center; align-items: center; }

.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);      /* 3 equal columns */
  grid-template-columns: 250px 1fr;           /* sidebar + fluid main */
  grid-template-rows: auto 1fr auto;          /* header / content / footer */
  gap: 20px;
}

/* THE responsive card grid — no media queries needed: */
.cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 20px;
}

.item { grid-column: 1 / 3; }     /* span columns 1–2 */
.item { grid-column: span 2; }    /* same thing */

/* named areas — layout you can read: */
.page {
  grid-template-areas: "header header" "sidebar main" "footer footer";
}
header { grid-area: header; }

/* Mobile-first: base styles = mobile, then enhance upward */
.container { padding: 16px; }

@media (min-width: 640px)  { .container { padding: 24px; } }
@media (min-width: 1024px) { .container { max-width: 1200px; margin-inline: auto; } }

@media (prefers-color-scheme: dark)   { /* dark mode */ }
@media (prefers-reduced-motion: reduce) { * { animation: none; } }
@media print { nav, footer { display: none; } }

/* Container queries — respond to the PARENT's width (modern) */
.card-wrap { container-type: inline-size; }
@container (min-width: 400px) { .card { flex-direction: row; } }

/* pseudo-CLASSES (:) — element states */
a:hover, a:focus-visible { color: royalblue; }
input:focus  { border-color: royalblue; }
input:disabled, input:invalid, input:checked { … }
li:first-child, li:last-child, li:nth-child(2n) { … }

/* pseudo-ELEMENTS (::) — generated content */
.quote::before { content: '"'; }
p::first-line  { font-weight: 700; }
::selection    { background: gold; }
input::placeholder { color: gray; }

/* decorative shapes without extra HTML: */
.badge::after {
  content: '';
  position: absolute; top: -4px; right: -4px;
  width: 10px; height: 10px; border-radius: 50%;
  background: red;
}

.btn {
  transition: transform 0.2s ease, box-shadow 0.2s ease;
  /* property | duration | timing-function | delay */
}
.btn:hover { transform: translateY(-2px) scale(1.02); }

transform: translate(10px, 20px);  /* move — GPU-accelerated, smooth */
transform: rotate(45deg);
transform: scale(1.5);
transform: skew(10deg);
transform: translateX(-50%);       /* half of OWN width — centering trick */

@keyframes float {
  0%   { transform: translateY(0); }
  50%  { transform: translateY(-12px); }
  100% { transform: translateY(0); }
}
.card {
  animation: float 3s ease-in-out infinite;
  /* name | duration | timing | iteration-count */
  animation-delay: 0.5s;
  animation-fill-mode: forwards;   /* keep the final state */
  animation-play-state: paused;    /* control from JS/hover */
}

box-shadow: 0 4px 16px rgba(0,0,0,0.25);      /* x y blur color */
box-shadow: inset 0 1px 0 rgba(255,255,255,.1);/* inner highlight */
box-shadow: 0 0 0 3px royalblue;               /* focus ring */
text-shadow: 0 2px 8px rgba(0,0,0,0.4);

filter: blur(4px) brightness(1.2) grayscale(50%) drop-shadow(0 2px 4px #000);
backdrop-filter: blur(16px);   /* frosted glass — blurs what's BEHIND */

border-radius: 12px;           /* 50% = circle */
opacity: 0.8;
clip-path: polygon(0 0, 100% 0, 100% 80%, 0 100%);  /* angled sections */

:root {
  --brand: #6d6ffb;
  --space: 16px;
}
.btn {
  background: var(--brand);
  padding: var(--space) calc(var(--space) * 2);
}
.dark-theme { --brand: #8b5cf6; }   /* theme switch = swap variables */

/* modern math functions */
width: min(90%, 1200px);            /* whichever is SMALLER */
width: max(300px, 50%);             /* whichever is LARGER  */
font-size: clamp(1rem, 2.5vw, 1.5rem);  /* min | preferred | max — fluid type */
height: calc(100vh - 64px);         /* mix units freely */
aspect-ratio: 16 / 9;               /* responsive video boxes */

overflow: visible | hidden | scroll | auto;
overflow-x: auto;                 /* horizontal scroll for tables on mobile */
scroll-behavior: smooth;          /* smooth anchor jumps */
scroll-margin-top: 80px;          /* anchor lands below your sticky navbar */
overscroll-behavior: contain;     /* stop scroll chaining in modals */

/* scroll snapping — carousels without JS */
.carousel { scroll-snap-type: x mandatory; overflow-x: auto; display: flex; }
.slide    { scroll-snap-align: center; flex: 0 0 100%; }

/* 1. Automatic — follow the OS setting */
:root { --bg: #fff; --text: #111; }
@media (prefers-color-scheme: dark) {
  :root { --bg: #0a0a10; --text: #f4f4f6; }
}
body { background: var(--bg); color: var(--text); }

/* 2. Manual toggle — JS adds a class, CSS swaps variables */
body.dark { --bg: #0a0a10; --text: #f4f4f6; }

// remember the choice
btn.onclick = () => {
  document.body.classList.toggle('dark');
  localStorage.setItem('theme', document.body.classList.contains('dark') ? 'dark' : 'light');
};