/* Chapter 03 follows the supplied reference; text and diagrams remain editable. */
.problem-reference {
  position: relative;
  isolation: isolate;
  overflow: hidden;
  padding: clamp(36px, 2.5vw, 50px) 0 clamp(44px, 3.6vw, 72px);
  color: #070d25;
  background: radial-gradient(ellipse at 53% 53%, #e9f4ff 0, transparent 63%), linear-gradient(125deg, #f8fbff, #f3f9ff 52%, #f8fbff);
}
.problem-reference::before, .problem-reference::after { content: ''; position: absolute; z-index: -1; pointer-events: none; border-radius: 50%; background: linear-gradient(135deg, #deecff7a, #e4f1ff26); }
.problem-reference::before { width: 480px; height: 480px; right: -90px; top: -120px; }
.problem-reference::after { width: 410px; height: 410px; bottom: -278px; left: 55%; box-shadow: -1150px -15px 0 0 #deecff55; }
.problem-reference .problem-shell { width: 91%; max-width: 1820px; margin-left: 5.2%; }
.problem-reference .problem-reference-layout { display: grid; grid-template-columns: 42.75% minmax(0, 1fr); gap: 2.15%; align-items: start; }
.problem-reference .problem-reference-layout > * { min-width: 0; }
.problem-reference .problem-kicker { color: #506692; font-size: 13px; font-weight: 500; line-height: 1.7; letter-spacing: .16em; }
.problem-reference h2 { margin: 30px 0 0; max-width: none; font-size: clamp(38px, 3.45vw, 68px); font-weight: 700; line-height: 1.34; letter-spacing: -.025em; text-wrap: initial; }
.problem-reference h2 span { display: block; }
.problem-reference .problem-intro { margin-top: 28px; color: #566c96; font-size: clamp(17px, 1.2vw, 23px); line-height: 1.55; }
.problem-reference .problem-intro p + p { margin-top: 20px; }
.problem-reference .problem-focus { position: relative; width: calc(100% + 22px); margin: 38px 0 0 -22px; padding: 23px 22px 22px 50px; border: 1px solid #fff; border-radius: 19px; background: #ffffffa8; box-shadow: 0 12px 30px #ccddf51c; }
.problem-reference .problem-focus::before { content: ''; position: absolute; top: 24px; left: 22px; width: 4px; height: 22px; border-radius: 2px; background: #2769ff; }
.problem-reference .problem-focus-label { color: #526fa6; font-size: 12px; line-height: 1.7; letter-spacing: .15em; }
.problem-reference .problem-focus-copy { margin-top: 10px; color: #546b95; font-size: clamp(15px, 1.1vw, 21px); line-height: 1.6; text-wrap: initial; }
.problem-reference .problem-focus h3 { margin-top: 7px; color: #071846; font-size: clamp(19px, 1.58vw, 31px); font-weight: 700; line-height: 1.45; letter-spacing: -.015em; text-wrap: initial; }
.problem-reference .problem-phrase { white-space: nowrap; }
.problem-reference .problem-principles { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 55px; width: calc(100% + 52px); margin: 42px 0 0 -22px; }
.problem-reference .problem-principles li { position: relative; display: flex; align-items: center; gap: 13px; min-width: 0; padding: 10px 13px; border: 1px solid #fff; border-radius: 17px; background: #ffffffa1; }
.problem-reference .problem-principles li:not(:last-child)::after { content: ''; position: absolute; top: 50%; left: calc(100% + 17px); width: 23px; height: 1px; background: #b7d0ff; }
.problem-reference .principle-icon { flex: 0 0 auto; display: grid; place-items: center; width: 52px; height: 52px; border: 1px solid #fff; border-radius: 50%; color: #2670ff; background: linear-gradient(145deg, #fff, #edf5ff); }
.problem-reference .principle-icon svg { width: 31px; height: 31px; }
.problem-reference .problem-principles h4 { font-size: clamp(16px, 1.15vw, 22px); line-height: 1.5; font-weight: 700; }
.problem-reference .problem-principles p { margin-top: 2px; color: #58729e; font-size: clamp(11px, .83vw, 16px); line-height: 1.6; white-space: nowrap; }
.problem-reference .problem-cards-column { position: relative; padding-top: 68px; }
.problem-reference .problem-handnote { position: absolute; top: -5px; right: 0; display: flex; align-items: center; gap: 8px; color: #8eaff0; font-family: 'Segoe Print', 'Comic Sans MS', cursive; font-size: 19px; line-height: 1.2; transform: rotate(-9deg); }
.problem-reference .problem-handnote svg { width: 44px; height: 32px; }
.problem-reference .problem-cards { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 20px 18px; }
.problem-reference .problem-card { position: relative; min-width: 0; min-height: clamp(270px, 17.35vw, 347px); padding: 33px 29px 54px; overflow: hidden; border: 1px solid #fff; border-radius: 19px; background: linear-gradient(125deg, #ffffffec, #ffffff9c); box-shadow: 0 10px 28px #cddff21b; }
.problem-reference .problem-card-number { position: relative; z-index: 1; display: flex; align-items: center; gap: 13px; color: #628df0; font-size: 14px; line-height: 1.7; font-variant-numeric: tabular-nums; }
.problem-reference .problem-card-number::after { content: ''; width: 36px; height: 1px; background: #b9d0ff; }
.problem-reference .problem-card h3 { position: relative; z-index: 1; margin-top: 23px; font-size: clamp(21px, 1.52vw, 30px); font-weight: 700; line-height: 1.45; text-wrap: initial; }
.problem-reference .problem-card-copy { position: relative; z-index: 1; max-width: 285px; width: 79%; margin-top: 24px; color: #556b94; font-size: clamp(15px, 1.06vw, 21px); line-height: 1.55; text-wrap: pretty; }
.problem-reference .problem-card-caption { position: absolute; left: 29px; bottom: 17px; margin: 0; color: #58729e; font-size: clamp(10px, .68vw, 13px); line-height: 1.6; letter-spacing: .12em; }
.problem-reference .problem-miniature { position: absolute; top: 26px; right: 8px; width: 46%; max-width: 200px; color: #2770ff; }
.problem-reference .problem-miniature svg { width: 100%; height: auto; overflow: visible; }
.problem-reference .problem-card:nth-child(2) .problem-miniature { right: 5px; top: 25px; width: 44%; }
.problem-reference .problem-card:nth-child(3) .problem-miniature { top: 28px; right: 15px; width: 38%; }
.problem-reference .problem-card:nth-child(4) .problem-miniature { top: 21px; right: 12px; width: 39%; }
.problem-reference .problem-card:nth-child(3) .problem-card-copy { width: 84%; }
.problem-reference .problem-echo { display: flex; align-items: flex-start; justify-content: flex-end; gap: 32px; margin: 50px 38px 0 0; color: #91acd9; font-size: 10px; line-height: 1.8; letter-spacing: .13em; }
.problem-reference .problem-echo::before { content: ''; width: 110px; height: 1px; margin-top: 17px; background: #bdd4ff; }
@media (min-width: 2001px) { .problem-reference .problem-shell { margin-inline: auto; } }
@media (max-width: 1450px) {
  .problem-reference .problem-focus { padding-left: 38px; }
  .problem-reference .problem-focus::before { left: 17px; }
  .problem-reference .problem-focus-copy { font-size: 15px; }
  .problem-reference .problem-principles { gap: 30px; width: calc(100% + 32px); }
  .problem-reference .problem-principles li { gap: 8px; padding: 11px 9px; }
  .problem-reference .problem-principles li:not(:last-child)::after { left: calc(100% + 8px); width: 16px; }
  .problem-reference .principle-icon { width: 40px; height: 40px; }
  .problem-reference .principle-icon svg { width: 26px; height: 26px; }
  .problem-reference .problem-card { padding-inline: 23px; }
  .problem-reference .problem-card-caption { left: 23px; }
  .problem-reference .problem-card-copy { width: 88%; }
  .problem-reference .problem-miniature { opacity: .75; }
}
@media (max-width: 1150px) {
  .problem-reference { padding: 40px 0 48px; }
  .problem-reference .problem-shell { width: calc(100% - 80px); margin-inline: auto; }
  .problem-reference .problem-reference-layout { grid-template-columns: 1fr; gap: 38px; }
  .problem-reference h2 { margin-top: 24px; font-size: clamp(38px, 5.2vw, 55px); }
  .problem-reference .problem-intro { font-size: 18px; }
  .problem-reference .problem-focus { width: 100%; margin: 30px 0 0; padding: 22px 26px 23px 45px; }
  .problem-reference .problem-focus h3 { font-size: 25px; }
  .problem-reference .problem-focus-copy { font-size: 17px; }
  .problem-reference .problem-principles { width: 100%; margin: 24px 0 0; gap: 40px; }
  .problem-reference .problem-principles li { padding: 12px 16px; gap: 13px; }
  .problem-reference .problem-principles p { font-size: 13px; }
  .problem-reference .problem-principles li:not(:last-child)::after { left: calc(100% + 12px); }
  .problem-reference .problem-cards-column { padding-top: 40px; }
  .problem-reference .problem-handnote { top: -8px; font-size: 17px; }
  .problem-reference .problem-card { min-height: 285px; padding: 28px 26px 52px; }
  .problem-reference .problem-card h3 { font-size: 24px; }
  .problem-reference .problem-card-copy { width: 84%; font-size: 17px; }
  .problem-reference .problem-card-caption { left: 26px; font-size: 11px; }
  .problem-reference .problem-miniature { opacity: .9; }
  .problem-reference .problem-echo { margin-top: 26px; margin-right: 8px; }
}
@media (max-width: 700px) {
  .problem-reference { padding: 32px 0 36px; }
  .problem-reference .problem-shell { width: calc(100% - 40px); }
  .problem-reference .problem-kicker { font-size: 12px; }
  .problem-reference h2 { margin-top: 22px; font-size: clamp(27px, 7.2vw, 42px); line-height: 1.45; letter-spacing: -.035em; }
  .problem-reference .problem-intro { margin-top: 24px; font-size: 15px; line-height: 1.75; }
  .problem-reference .problem-intro p + p { margin-top: 16px; }
  .problem-reference .problem-focus { margin-top: 28px; padding: 21px 19px 22px 29px; border-radius: 16px; }
  .problem-reference .problem-focus::before { left: 12px; top: 22px; height: 19px; width: 3px; }
  .problem-reference .problem-focus-label { font-size: 11px; }
  .problem-reference .problem-focus-copy { font-size: 14px; }
  .problem-reference .problem-focus h3 { margin-top: 10px; font-size: 20px; line-height: 1.55; }
  .problem-reference .problem-principles { grid-template-columns: 1fr; gap: 19px; }
  .problem-reference .problem-principles li { padding: 12px 16px; gap: 15px; }
  .problem-reference .problem-principles li:not(:last-child)::after { top: auto; bottom: -16px; left: 36px; width: 1px; height: 13px; }
  .problem-reference .problem-principles li > div { display: flex; gap: 18px; align-items: baseline; }
  .problem-reference .problem-principles h4 { font-size: 17px; }
  .problem-reference .problem-principles p { font-size: 13px; }
  .problem-reference .problem-reference-layout { gap: 32px; }
  .problem-reference .problem-cards-column { padding-top: 44px; }
  .problem-reference .problem-handnote { font-size: 15px; top: -2px; right: 4px; }
  .problem-reference .problem-cards { grid-template-columns: 1fr; gap: 16px; }
  .problem-reference .problem-card { padding: 24px 23px 50px; min-height: 270px; border-radius: 17px; }
  .problem-reference .problem-card h3 { margin-top: 19px; font-size: 23px; }
  .problem-reference .problem-card-copy { margin-top: 22px; width: 88%; font-size: 15px; line-height: 1.7; }
  .problem-reference .problem-card-caption { left: 23px; bottom: 17px; font-size: 10px; }
  .problem-reference .problem-card .problem-miniature { top: 12px; right: 12px; width: 35%; opacity: .8; }
  .problem-reference .problem-card:nth-child(3) .problem-card-copy { width: 90%; }
  .problem-reference .problem-echo { gap: 18px; margin: 26px 4px 0; }
  .problem-reference .problem-echo::before { width: 60px; }
}