Modern CSS & Best Practices

/* OLD WAY — repetitive */
h1 a, h2 a, h3 a, h4 a, h5 a, h6 a {
  color: inherit;
  text-decoration: none;
}

/* MODERN WAY — :is() groups them */
:is(h1, h2, h3, h4, h5, h6) a {
  color: inherit;
  text-decoration: none;
}

/* Combine contexts easily */
:is(.card, .modal, .sidebar) p {
  font-size: 0.9rem;
  color: var(--txt2);
}

/* :where() has 0 specificity — easy to override */
:where(ul, ol) {
  list-style: none;
  padding: 0;
  margin: 0;
}

/* This single-class rule easily overrides the :where() above */
.my-list {
  list-style: disc;  /* wins because .class > :where() */
}

/* Great for design-system base resets */
:where(button, input, select, textarea) {
  font-family: inherit;
  font-size: 1rem;
}

/* Style a card differently if it contains an image */
.card:has(img) {
  padding: 0;           /* remove padding when image present */
}

/* Style a form when any input is invalid */
form:has(:invalid) {
  border: 2px solid red;
}

/* Style a label when its checkbox is checked */
.toggle:has(input:checked) {
  background: var(--acc);
  color: white;
}

/* Layout change when sidebar is present */
body:has(.sidebar) .main {
  margin-left: 280px;    /* only when sidebar exists in DOM */
}

/* Step 1: declare a container on the PARENT */
.card-wrapper {
  container-type: inline-size;  /* tracks width */
  container-name: card;          /* optional name */
}

/* Step 2: query the container from CHILD styles */
@container card (min-width: 400px) {
  .product-card {
    display: grid;
    grid-template-columns: 1fr 1fr;
  }
  .product-card img {
    height: 100%;
    object-fit: cover;
  }
}

/* Without a name, queries apply to nearest containment ancestor */
.sidebar-widget {
  container-type: inline-size;
}

@container (max-width: 250px) {
  .sidebar-widget .icon-label {
    display: none;  /* hide text in tiny sidebars */
  }
}

/* Native CSS nesting — no Sass needed! */
.card {
  background: white;
  border-radius: 12px;
  padding: 1.5rem;

  /* Nested child selector */
  & .card-title {
    font-size: 1.2rem;
    font-weight: 700;
  }

  /* Nested hover state */
  &:hover {
    box-shadow: 0 8px 32px rgba(0,0,0,.15);
    transform: translateY(-2px);
  }

  /* Nested media query */
  @media (max-width: 600px) {
    padding: 1rem;
  }

  /* Nested modifier class */
  &.card--featured {
    border: 2px solid gold;
  }
}

/* 1. Use will-change SPARINGLY for animated elements */
.animated-card {
  will-change: transform;  /* hints browser to composite this layer */
}
/* ⚠️ Don't apply to everything — it uses GPU memory */

/* 2. Prefer transform & opacity for animations (GPU-accelerated) */
.slide-in {
  transform: translateX(-100%);  /* ✅ GPU */
  opacity: 0;                    /* ✅ GPU */
}
/* Avoid animating: width, height, top, left, margin (cause reflow) */

/* 3. Use content-visibility for off-screen sections */
.below-fold-section {
  content-visibility: auto;        /* skip rendering until visible */
  contain-intrinsic-size: 0 500px; /* estimated height hint */
}

/* 4. Limit selector depth — shallow is faster */
.nav-item { } /* ✅ fast */
/* header nav ul li a span { } — ❌ slow, avoid */

/* 5. Use logical properties for international support */
.box {
  margin-inline-start: 1rem;  /* left in LTR, right in RTL */
  padding-block: 0.5rem;     /* top + bottom padding */
}

/* 1. Never remove focus outlines — restyle them instead */
:focus-visible {
  outline: 2px solid var(--acc);
  outline-offset: 3px;
}

/* 2. Respect user motion preferences */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

/* 3. Respect user color-scheme preference */
@media (prefers-color-scheme: light) {
  :root {
    --bg: #ffffff;
    --txt: #111111;
  }
}

/* 4. Visually hidden but accessible to screen readers */
.sr-only {
  position: absolute;
  width: 1px; height: 1px;
  padding: 0; margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  white-space: nowrap;
  border: 0;
}

/* 5. Minimum touch target sizes */
.btn {
  min-height: 44px;  /* WCAG 2.5.5 recommended */
  min-width: 44px;
}

/* @layer — explicit cascade ordering (no !important needed) */
@layer reset, base, components, utilities;

@layer reset {
  *, *::before, *::after { box-sizing: border-box; }
}

@layer base {
  body { font-family: var(--ff-b); }
}

@layer components {
  .btn { padding: 0.5rem 1rem; border-radius: 6px; }
}

@layer utilities {
  /* Utilities always win over components in the same layer order */
  .mt-4 { margin-top: 1rem; }
  .hidden { display: none; }
}