CSS3: From Box Model to Professional Architecture

/* 1. Tag Selector — targets ALL elements of that type */
p { color: #555; }

/* 2. Class Selector — targets elements with class="card" */
.card { border-radius: 8px; }

/* 3. ID Selector — targets ONE unique element with id="hero" */
#hero { background: #0d1117; }

/* Descendant: any .title inside .card (any depth) */
.card .title { color: white; }

/* Child: only DIRECT .title children of .card */
.card > .title { font-size: 1.2rem; }

/* Adjacent sibling: the first p right after an h2 */
h2 + p { margin-top: 0; }

/* General sibling: ALL p elements after h2 */
h2 ~ p { color: #8b949e; }

/* Attribute selector: target inputs by type */
input[type="email"] { border-color: #3b82f6; }

/* Attribute contains: any href with "google" */
a[href*="google"] { color: #34d399; }

.card {
    /* Content size */
    width: 300px;

    /* Padding: space inside */
    padding: 20px;

    /* Border: visible edge */
    border: 1px solid #30363d;

    /* Margin: space outside */
    margin: 16px;
}

/* CRITICAL: Always add this to your CSS reset.
   It makes width include padding and border — 
   avoiding confusing layout math. */
*, *::before, *::after {
    box-sizing: border-box;
}

body {
    font-family: 'Inter', sans-serif;
    font-size: 16px;
    line-height: 1.7;
    color: #e6edf3;
    background-color: #0d1117;
}

h1 {
    font-size: 2.5rem;
    font-weight: 700;
    letter-spacing: -0.02em;
}

.navbar {
    display: flex;
    justify-content: space-between; /* Space between logo and links */
    align-items: center;            /* Vertically center everything */
    gap: 1.5rem;                    /* Space between flex children */
}

/* Center anything perfectly */
.hero {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
}

/* 3-column responsive card grid */
.card-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr); /* 3 equal columns */
    gap: 1.5rem;
}

/* Named grid areas for complex page layouts */
.page-layout {
    display: grid;
    grid-template-areas:
        "header header"
        "sidebar main"
        "footer footer";
    grid-template-columns: 280px 1fr;
}

.site-header { grid-area: header; }
.sidebar     { grid-area: sidebar; }
.main        { grid-area: main; }
.site-footer { grid-area: footer; }

/* Mobile styles first (default) */
.card-grid {
    display: grid;
    grid-template-columns: 1fr; /* 1 column on mobile */
}

/* Then scale UP for larger screens */
@media (min-width: 600px) {
    .card-grid {
        grid-template-columns: 1fr 1fr; /* 2 columns on tablet */
    }
}

@media (min-width: 1024px) {
    .card-grid {
        grid-template-columns: repeat(3, 1fr); /* 3 on desktop */
    }
}

/* clamp(minimum, preferred, maximum) */
/* This heading scales fluidly — no media query needed! */
h1 {
    font-size: clamp(1.75rem, 5vw + 1rem, 3.5rem);
}

/* Smooth hover transition */
.btn {
    background: #3b82f6;
    transition: background 0.2s ease, transform 0.2s ease;
}
.btn:hover {
    background: #2563eb;
    transform: translateY(-2px);
}

/* Keyframe animation */
@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.hero-text {
    animation: fadeInUp 0.6s ease forwards;
}

:root {
    --primary:    #3b82f6;
    --bg:         #0d1117;
    --text:       #e6edf3;
    --radius:     8px;
    --spacing-md: 1.5rem;
}

/* Now use them everywhere */
.card {
    background: var(--bg);
    border-radius: var(--radius);
    padding: var(--spacing-md);
}

/* Change the whole theme by updating ONE value */
[data-theme="light"] {
    --bg:   #ffffff;
    --text: #1a1a1a;
}

/* Pseudo-classes — element state */
a:hover   { color: #60a5fa; }
a:visited { color: #a78bfa; }
li:nth-child(2) { font-weight: bold; }
input:focus { outline: 2px solid #3b82f6; }

/* Pseudo-elements — inject content via CSS */
.card::before {
    content: '';
    display: block;
    height: 4px;
    background: linear-gradient(90deg, #3b82f6, #8b5cf6);
    border-radius: 4px 4px 0 0;
}

/* Block: standalone component */
.card { }

/* Element: part of the block */
.card__title { font-size: 1.25rem; }
.card__image { width: 100%; }
.card__body  { padding: 1rem; }

/* Modifier: a variation of the block */
.card--featured { border: 2px solid #3b82f6; }
.card--compact  { padding: 0.5rem; }

// Variables
$primary: #3b82f6;
$radius:  8px;

// Nesting (no need to repeat parent selector)
.card {
    border-radius: $radius;

    &__title {
        font-size: 1.25rem;
        color: $primary;
    }

    &:hover {
        box-shadow: 0 8px 24px rgba(0,0,0,0.2);
    }
}

// Reusable mixin
@mixin flex-center {
    display: flex;
    justify-content: center;
    align-items: center;
}

.hero { @include flex-center; }