:root{--teal:#147b7a;--teal-dark:#115b5b;--text:#203540;--muted:#6f7e85;--line:#dfe6e6;--light:#f4f8f7;--white:#fff}
*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;color:var(--text);background:#f9fbfb;font:15px/1.5 system-ui,-apple-system,"Segoe UI",Arial,sans-serif}button,input{font:inherit}button{cursor:pointer}button:focus-visible,input:focus-visible{outline:3px solid #efba64;outline-offset:2px}button:disabled{cursor:default}img{display:block;max-width:100%}[hidden]{display:none!important}
.header{background:white;border-bottom:1px solid var(--line)}.header-inner{max-width:1260px;margin:auto;min-height:73px;padding:13px 25px;display:flex;justify-content:space-between;align-items:center;gap:16px}.brand{display:flex;align-items:center;gap:11px}.brand-symbol{display:grid;place-items:center;background:#e5f2f0;color:var(--teal);width:40px;height:40px;border-radius:9px;font-size:33px;font-weight:700;line-height:1}.brand strong,.brand span{display:block}.brand strong{font-size:16px;line-height:1.2}.brand div>span{font-size:11px;color:var(--muted);margin-top:3px}.credit-top{color:#849097;font-size:11px}.credit-top strong{color:#46616b}
.page{max-width:1260px;margin:auto;padding:30px 25px 38px}.intro h1{font-size:28px;line-height:1.2;letter-spacing:-.035em;margin:0 0 7px}.intro p{margin:0;color:var(--muted);font-size:13px;max-width:840px}.tabs{display:flex;gap:8px;border-bottom:1px solid #cdd9d8;margin-top:28px;padding-bottom:0;overflow-x:auto}.tab{appearance:none;border:0;border-bottom:3px solid transparent;background:transparent;color:#6e8086;padding:12px 17px 11px;font-size:13px;font-weight:700;white-space:nowrap}.tab:hover{color:var(--teal)}.tab.active{border-bottom-color:var(--teal);color:var(--teal-dark)}.tab span{font-size:10px;border-radius:20px;padding:2px 6px;background:#e8eeee;margin-left:5px;color:#68837e}.tab.active span{background:#deeeea;color:var(--teal-dark)}
.workspace{display:grid;grid-template-columns:275px minmax(0,1fr);align-items:start;gap:18px;margin-top:18px}.topic-panel,.review-panel{background:white;border:1px solid var(--line);border-radius:10px}.topic-panel{overflow:hidden}.topic-header{padding:18px 15px 14px;border-bottom:1px solid #e8eeee}.topic-header h2{font-size:13px;margin:0 0 13px}.search{height:35px;display:flex;align-items:center;gap:6px;border:1px solid #dbe5e4;border-radius:7px;padding:0 9px}.search span{font-size:22px;line-height:1;color:#8d9b9f}.search input{border:0;outline:0;width:100%;min-width:0;background:transparent;font-size:12px;color:var(--text)}.topic-list{padding:7px}.topic-button{display:flex;text-align:left;align-items:center;gap:11px;width:100%;padding:10px;border:1px solid transparent;border-radius:7px;background:white;color:var(--text)}.topic-button:hover{background:#f2f8f6}.topic-button.active{background:#e8f4f0;border-color:#b5dcd1}.topic-thumbnail{flex:none;width:60px;height:47px;background:#e8efed;border:1px solid #e2e8e5;border-radius:5px;overflow:hidden}.topic-thumbnail img{height:100%;width:100%;object-fit:contain;pointer-events:none}.topic-copy{min-width:0}.topic-copy strong{display:block;font-size:12px;line-height:1.3;color:#31474f;font-weight:700}.topic-copy small{display:block;color:#8b979c;font-size:10px;margin-top:4px}.no-topics{padding:20px;color:var(--muted);font-size:12px}
.review-panel{min-width:0;padding:24px 26px 23px}.section-tag{text-transform:uppercase;font-size:10px;font-weight:800;letter-spacing:.09em;color:var(--teal)}.review-heading h2{font-size:24px;line-height:1.25;letter-spacing:-.03em;margin:6px 0}.review-heading p{font-size:13px;color:var(--muted);margin:0 0 19px}.viewer-toolbar{display:flex;align-items:center;justify-content:space-between;gap:10px;margin-bottom:7px;font-size:11px;color:#84959a}.zoom-actions{display:flex;align-items:center;gap:5px}.zoom-actions>span{margin-right:6px}.zoom-actions button{font-size:12px;min-width:27px;border:1px solid #d7e2e0;border-radius:5px;background:#fff;color:#305860;padding:4px 8px}.zoom-actions button:hover{background:#e7f4ef}.zoom-actions output{font-size:11px;text-align:center;min-width:40px;color:#54706f}.media-frame{border:1px solid #dfe6e3;border-radius:8px;background:#eef2f1;overflow:auto;min-height:300px;max-height:630px}.media-inner{width:100%;min-width:100%;position:relative}.media-inner img{width:100%;height:auto;max-width:none;user-select:none;-webkit-user-drag:none}.watermark{position:absolute;bottom:8px;right:8px;background:#16323bcf;border-radius:4px;color:white;padding:5px 8px;letter-spacing:.01em;font-size:9px;font-weight:700;pointer-events:none;user-select:none}.hint{font-size:11px;color:#93a1a3;margin-top:7px}.notes{display:grid;grid-template-columns:1.2fr 1fr;gap:24px;border-top:1px solid #e7edec;margin-top:21px;padding-top:19px}.notes h3{font-size:13px;margin:0 0 9px;color:#2f535b}.steps ol{margin:0;padding-left:22px}.steps li{font-size:12px;line-height:1.7;padding-left:3px;margin-bottom:6px;color:#465c65}.steps li::marker{font-weight:700;color:var(--teal)}.side-notes section{padding:13px 15px;border-radius:7px}.remember{background:#edf7f3;border:1px solid #d6e8e0}.remember p{font-size:12px;line-height:1.65;color:#3b655b;margin:0}.self-check{margin-top:12px;background:#f8fafb;border:1px solid #e2e8e8}.self-check p{font-size:12px;margin:0 0 12px;color:#566871;line-height:1.65}.self-check button{background:white;border:1px solid #a6cfc5;color:var(--teal-dark);font-size:11px;font-weight:700;border-radius:5px;padding:6px 10px}.self-check .answer{margin-top:10px;margin-bottom:0;border-left:3px solid var(--teal);padding:6px 10px;background:#eaf7f0;color:#2d5d51;border-radius:3px}.disclaimer{font-size:11px;text-align:center;color:#8d999f;margin:24px 0 0}.footer{background:#eef3f2;border-top:1px solid #dfe8e5;padding:18px 25px;color:#8a999d;font-size:10px;display:flex;justify-content:center;gap:10px 28px;flex-wrap:wrap}
@media(max-width:820px){.page{padding:25px 16px 30px}.workspace{grid-template-columns:1fr}.topic-header{display:flex;align-items:center;justify-content:space-between;gap:16px}.topic-header h2{margin:0}.search{width:230px}.topic-list{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:4px}.review-panel{padding:18px}.notes{gap:16px}.media-frame{max-height:540px}}
@media(max-width:570px){.header-inner{min-height:65px;padding:12px 16px}.brand strong{font-size:14px}.credit-top{font-size:9px;text-align:right;max-width:120px}.intro h1{font-size:24px}.tabs{gap:1px}.tab{padding:12px 10px;font-size:11px}.tab span{display:none}.topic-header{display:block}.topic-header h2{margin-bottom:9px}.search{width:100%}.topic-list{grid-template-columns:1fr}.topic-button{padding:7px}.topic-thumbnail{width:55px;height:40px}.review-panel{padding:16px 12px}.review-heading h2{font-size:21px}.viewer-toolbar{flex-wrap:wrap}.media-frame{max-height:450px;min-height:180px}.notes{grid-template-columns:1fr;gap:10px}.watermark{font-size:7px}.footer{display:block;text-align:center}.footer span{display:block;margin-bottom:4px}}

/* No GIFs load or play until the student explicitly selects a topic. */
.topic-thumbnail{display:grid;place-items:center;width:36px;height:36px;background:#ecf3f1;color:#4e8e85;border-color:#dce9e5;font-weight:800;font-size:11px;font-variant-numeric:tabular-nums}
.empty-panel{min-height:510px;background:#fff;border:1px solid var(--line);border-radius:10px;display:flex;align-items:center;justify-content:center;flex-direction:column;text-align:center;padding:40px 24px;color:var(--muted)}
.empty-icon{display:grid;place-items:center;width:56px;height:56px;border-radius:50%;background:#e9f5f1;color:var(--teal);font-size:19px;padding-left:3px;margin-bottom:15px}
.empty-panel h2{color:#334b52;font-size:19px;margin:0 0 7px}
.empty-panel p{max-width:370px;font-size:13px;margin:0;line-height:1.7}
@media(max-width:820px){.empty-panel{min-height:300px}}

/* Related circuit cases sit inside one expandable topic. */
.topic-group{margin:4px 0;border:1px solid #dfeae7;border-radius:7px;overflow:hidden}
.group-toggle{display:flex;align-items:center;justify-content:space-between;gap:10px;width:100%;border:0;background:#f1f8f5;color:#235e58;padding:13px 12px;text-align:left;font-size:12px;font-weight:800}
.group-toggle:hover{background:#e7f3ee}
.group-arrow{font-size:17px;line-height:1;transition:transform .15s}
.group-toggle[aria-expanded="true"] .group-arrow{transform:rotate(180deg)}
.group-children{padding:5px 6px 7px 11px;background:#fbfdfc;border-top:1px solid #e1ece7}
.topic-child{padding:9px 7px;gap:8px}
.topic-child .topic-copy strong{font-weight:600;font-size:11px}
.case-marker{width:14px;flex:none;color:var(--teal);font-size:17px;line-height:1;text-align:center}