* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: "Segoe UI", Arial, sans-serif;
  background: linear-gradient(135deg, #1a1a2e 0%, #16213e 100%);
  color: #eee;
  padding: 40px 20px;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
}

h1 {
  text-align: center;
  margin-bottom: 10px;
}

.subtitle {
  text-align: center;
  color: #aaa;
  margin-bottom: 40px;
}

.section {
  background-color: #0f3460;
  border-radius: 16px;
  padding: 25px;
  margin-bottom: 30px;
}

.section-title {
  font-size: 24px;
  font-weight: bold;
  margin-bottom: 15px;
  color: #4a90e2;
  border-left: 4px solid #4a90e2;
  padding-left: 15px;
}

.section-description {
  color: #aaa;
  margin-bottom: 25px;
  font-size: 14px;
  padding-left: 19px;
}

.demo-box {
  background-color: #2a3a5a;
  border-radius: 12px;
  padding: 20px;
  margin: 20px 0;
}

.code {
  background-color: #0a0a1a;
  padding: 15px;
  border-radius: 8px;
  font-family: "Courier New", monospace;
  font-size: 13px;
  color: #82b1ff;
  overflow-x: auto;
  margin-top: 15px;
}

hr {
  border-color: #2a3a5a;
  margin: 30px 0;
}

/* ===================================
           DEMO 1: :hover (already covered, but review)
           =================================== */
.hover-box {
  background-color: #e94560;
  padding: 20px;
  text-align: center;
  border-radius: 8px;
  transition: all 0.3s ease;
  cursor: pointer;
}

.hover-box:hover {
  background-color: #c73b54;
  transform: scale(1.02);
}

/* ===================================
           DEMO 2: :focus (already covered)
           =================================== */
.focus-input {
  width: 100%;
  padding: 12px;
  border: 2px solid #2a3a5a;
  border-radius: 8px;
  background-color: #1a1a2e;
  color: white;
  transition: all 0.3s ease;
}

.focus-input:focus {
  outline: none;
  border-color: #4a90e2;
  box-shadow: 0 0 0 3px rgba(74, 144, 226, 0.3);
}

/* ===================================
           DEMO 3: :first-child and :last-child
           =================================== */
.list-demo {
  list-style: none;
}

.list-demo li {
  background-color: #2a3a5a;
  margin: 5px 0;
  padding: 10px;
  border-radius: 6px;
}

.list-demo li:first-child {
  background-color: #4a90e2;
  font-weight: bold;
}

.list-demo li:last-child {
  background-color: #e94560;
  font-weight: bold;
}

/* ===================================
           DEMO 4: :nth-child()
           =================================== */
.nth-demo {
  list-style: none;
}

.nth-demo li {
  background-color: #2a3a5a;
  margin: 5px 0;
  padding: 10px;
  border-radius: 6px;
}

/* Every even item (2nd, 4th, 6th...) */
.nth-demo li:nth-child(even) {
  background-color: #4a90e2;
}

/* Every odd item (1st, 3rd, 5th...) */
.nth-demo-2 li:nth-child(odd) {
  background-color: #e94560;
}

/* Every 3rd item */
.nth-demo-3 li:nth-child(3n) {
  background-color: #2ecc71;
}

/* Specific item (3rd child) */
.nth-demo-4 li:nth-child(3) {
  background-color: #f39c12;
  font-weight: bold;
}

/* ===================================
           DEMO 5: :first-of-type and :last-of-type
           =================================== */
.article-demo {
  background-color: #2a3a5a;
  padding: 15px;
  border-radius: 8px;
}

.article-demo h3:first-of-type {
  color: #4a90e2;
}

.article-demo p:last-of-type {
  color: #2ecc71;
}

/* ===================================
           DEMO 6: :not() (exclude items)
           =================================== */
.not-demo li {
  background-color: #2a3a5a;
  margin: 5px 0;
  padding: 10px;
  border-radius: 6px;
}

/* Style all li EXCEPT the one with class "special" */
.not-demo li:not(.special) {
  opacity: 0.6;
}

.special {
  background-color: #e94560;
  font-weight: bold;
}

/* ===================================
           DEMO 7: :is() (group selectors)
           =================================== */
.is-demo h1,
.is-demo h2,
.is-demo h3 {
  color: #4a90e2;
}

/* Same as above but shorter */
.is-demo :is(h1, h2, h3) {
  color: #e94560;
}

/* ===================================
           DEMO 8: :where() (same as :is but lower specificity)
           =================================== */
.where-demo :where(h1, h2, h3) {
  color: #2ecc71;
}

/* ===================================
           DEMO 9: :checked (for checkboxes/radios)
           =================================== */
.checked-demo input {
  margin-right: 10px;
}

.checked-demo label {
  margin-right: 20px;
}

.checked-demo input:checked + label {
  color: #2ecc71;
  font-weight: bold;
}

/* ===================================
           DEMO 10: :disabled and :enabled
           =================================== */
.disabled-input:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.enabled-input:enabled {
  background-color: #2a3a5a;
  border-color: #4a90e2;
}

/* ===================================
           DEMO 11: :required and :optional
           =================================== */
.required-input:required {
  border-left: 4px solid #e94560;
}

.optional-input:optional {
  border-left: 4px solid #2ecc71;
}

/* ===================================
           DEMO 12: :valid and :invalid
           =================================== */
.valid-input:valid {
  border-color: #2ecc71;
}

.valid-input:invalid {
  border-color: #e94560;
}

footer {
  text-align: center;
  padding: 40px;
  color: #666;
}