*{box-sizing:border-box}
:root{
 --bg:#eef5f7;--panel:#fff;--text:#18323b;--muted:#607780;--brand:#167c83;--brand2:#0d5e64;
 --accent:#ef5b5b;--blue:#2b78a6;--gold:#d99a2b;--green:#2f8f63;--purple:#7658a8;
 --line:#d7e4e8;--soft:#f5fafb;--shadow:0 12px 30px rgba(31,72,83,.10);--radius:18px
}
html{scroll-behavior:smooth}
body{margin:0;font-family:Inter,ui-sans-serif,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;background:linear-gradient(180deg,#f7fbfc,var(--bg));color:var(--text);min-height:100vh}
button,input,select,textarea{font:inherit} button{cursor:pointer}
a{color:inherit}
.site-header{max-width:1240px;margin:auto;padding:22px 24px 8px;display:flex;justify-content:space-between;align-items:center;gap:16px}
.brand{display:flex;align-items:center;gap:10px;text-decoration:none;font-weight:900;color:var(--brand2);font-size:1.35rem}
.brand-badge{display:grid;place-items:center;width:38px;height:38px;border-radius:11px;background:var(--brand);color:#fff;font-size:.9rem}
.home-link{text-decoration:none;color:var(--brand2);font-weight:800;font-size:.92rem}
.page{max-width:1240px;margin:auto;padding:18px 24px 42px}
.panel{background:rgba(255,255,255,.97);border:1px solid var(--line);border-radius:var(--radius);box-shadow:var(--shadow)}
.hero{padding:28px 32px;display:flex;justify-content:space-between;align-items:center;gap:28px}
.hero h1{font-size:clamp(2rem,4vw,3.2rem);line-height:1.02;letter-spacing:-.04em;margin:.15rem 0 .65rem}
.hero p{margin:0;color:var(--muted);line-height:1.6;max-width:780px}
.eyebrow{text-transform:uppercase!important;letter-spacing:.09em;font-weight:900!important;font-size:.75rem!important;color:var(--brand)!important}
.badge-stack{display:flex;flex-wrap:wrap;gap:8px;justify-content:flex-end}
.badge{padding:8px 11px;border-radius:999px;background:#e9f6f6;color:var(--brand2);font-size:.8rem;font-weight:850;border:1px solid #cfe6e7}
.grid{display:grid;grid-template-columns:minmax(290px,.8fr) minmax(480px,1.55fr);gap:22px;margin-top:22px}
.controls,.workspace,.data-panel{padding:24px}
.section-title{display:flex;align-items:flex-start;gap:11px;margin-bottom:16px}
.step{width:31px;height:31px;border-radius:9px;background:var(--brand);color:#fff;display:grid;place-items:center;font-weight:900;flex:0 0 auto}
.section-title h2,.section-title h3{margin:0 0 3px}.section-title p{margin:0;color:var(--muted);font-size:.9rem}
label.control-label{display:block;font-weight:800;margin:14px 0 7px}
select,input[type=number],input[type=text]{width:100%;padding:11px 12px;border:1px solid #c8d8dc;border-radius:10px;background:#fff;color:var(--text)}
input[type=range]{width:100%;accent-color:var(--brand)}
.row{display:flex;gap:10px;align-items:center}.row>*{min-width:0}.row.wrap{flex-wrap:wrap}
.btn{border:0;border-radius:10px;padding:11px 14px;font-weight:850;background:var(--brand);color:white;box-shadow:0 6px 13px rgba(22,124,131,.16)}
.btn.secondary{background:#e9f6f6;color:var(--brand2);box-shadow:none}.btn.ghost{background:#f1f5f6;color:#536970;box-shadow:none}.btn.danger{background:#a94b53}.btn:disabled{opacity:.45;cursor:not-allowed}
.callout{border-left:4px solid var(--brand);padding:12px 14px;background:#f0f9f9;border-radius:9px;color:#526b73;line-height:1.5;font-size:.9rem}
.stat-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:10px}
.stat{padding:12px;border:1px solid var(--line);border-radius:12px;background:var(--soft)}
.stat span{display:block;color:var(--muted);font-size:.74rem;text-transform:uppercase;letter-spacing:.05em;font-weight:850}.stat strong{display:block;font-size:1.16rem;margin-top:3px}
.table-wrap{overflow:auto;border:1px solid var(--line);border-radius:12px}
table{width:100%;border-collapse:collapse;background:#fff}th,td{padding:10px 11px;border-bottom:1px solid #e2eaec;text-align:left;font-size:.88rem}th{background:#eef7f7;color:var(--brand2);font-size:.76rem;text-transform:uppercase;letter-spacing:.05em}
canvas{display:block;max-width:100%;width:100%;height:auto}
.graph-card{border:1px solid var(--line);background:#fbfdfd;border-radius:14px;padding:14px}
.graph-card h3{margin:0}.graph-card p{color:var(--muted);font-size:.85rem;margin:.25rem 0 .6rem}
.status{padding:11px 13px;border-radius:10px;background:#f3f7f8;color:#50666e;font-weight:750;line-height:1.4}
.status.good{background:#ecf8f1;color:#276b4b;border:1px solid #cce6d6}.status.warn{background:#fff7e7;color:#805c1c;border:1px solid #efdcb5}.status.bad{background:#fff0f0;color:#84434a;border:1px solid #f0cfd2}
footer{max-width:1240px;margin:auto;padding:0 24px 34px;color:#6d8087;font-size:.82rem;display:flex;justify-content:space-between;gap:12px}
.small{font-size:.82rem;color:var(--muted);line-height:1.5}
.muted{color:var(--muted)}
code.formula{font-family:"Segoe UI",Arial,sans-serif;font-weight:800}
@media(max-width:900px){.grid{grid-template-columns:1fr}.hero{flex-direction:column;align-items:flex-start}.badge-stack{justify-content:flex-start}.stat-grid{grid-template-columns:1fr 1fr}}
@media(max-width:600px){.site-header,.page,footer{padding-left:14px;padding-right:14px}.hero,.controls,.workspace,.data-panel{padding:18px}.site-header{align-items:flex-start;flex-direction:column}.stat-grid{grid-template-columns:1fr}.row{flex-wrap:wrap}}

.toolbar{display:grid;grid-template-columns:1fr 1fr auto;gap:14px;align-items:end}.toolbar-buttons{display:flex;gap:8px;padding-bottom:1px}
.periodic-shell{padding:22px}.table-header{display:flex;justify-content:space-between;gap:18px;align-items:flex-start;margin-bottom:12px}.table-header h2{margin:0}.table-header p{margin:.3rem 0 0;color:var(--muted);font-size:.88rem}.legend{display:flex;gap:12px;flex-wrap:wrap;font-size:.8rem;color:var(--muted)}.dot{display:inline-block;width:9px;height:9px;border-radius:50%;margin-right:4px}.dot.measured{background:var(--brand)}.dot.selected{background:var(--accent)}
.periodic-scroll{overflow:auto;padding-bottom:5px}.periodic-table{min-width:930px;display:grid;grid-template-columns:repeat(18,1fr);grid-template-rows:repeat(7,63px);gap:4px}
.element{position:relative;border:1px solid #cfdde0;border-radius:7px;background:#f8fbfc;min-width:45px;padding:4px;cursor:pointer;text-align:left;color:var(--text)}.element:hover,.element:focus{outline:2px solid #8ec4c7;z-index:2}.element.selected{outline:3px solid var(--accent);background:#fff3f3}.element .z{font-size:.58rem;color:#6b7f86}.element .sym{display:block;font-weight:950;font-size:1.05rem;line-height:1.05}.element .name{font-size:.52rem;color:#687d84;white-space:nowrap;overflow:hidden;text-overflow:ellipsis}.element .has-data{position:absolute;right:4px;top:4px;width:6px;height:6px;border-radius:50%;background:var(--brand)}
.element[data-cat="Alkali metal"],.element[data-cat="Alkaline earth metal"]{background:#f8f2e7}.element[data-cat="Halogen"],.element[data-cat="Nonmetal"]{background:#eef7f1}.element[data-cat="Noble gas"]{background:#f0f1fa}.element[data-cat="Transition metal"]{background:#eef4f8}.element[data-cat="Metalloid"]{background:#f5f0f8}.element[data-cat="Lanthanide"],.element[data-cat="Actinide"]{background:#f8f1eb}
.placeholder{display:grid;place-items:center;border:1px dashed #c9d8dc;border-radius:7px;color:#71858c;font-size:.65rem;background:#f7fafb}
.series-row{max-width:800px;margin:7px auto 0;display:grid;grid-template-columns:70px repeat(15,1fr);gap:4px;min-width:0}.series-label{display:flex;align-items:center;justify-content:flex-end;padding-right:6px;color:var(--muted);font-size:.7rem;font-weight:800}.series-row .element{height:55px}
.element-panel{display:flex;align-items:center;gap:18px;border:1px solid var(--line);border-radius:14px;padding:15px;background:#f8fbfc;margin-bottom:12px}.big-symbol{width:74px;height:74px;border-radius:15px;background:var(--brand);color:white;display:grid;place-items:center;font-size:2rem;font-weight:950}.element-panel h3{font-size:1.35rem;margin:0}.element-panel p{margin:.25rem 0 0}
.challenge-box{margin-top:18px;border-top:1px solid var(--line);padding-top:16px}.challenge-box h3{margin:0}.challenge-box p{color:var(--muted)}
.choice-btn{border:1px solid #bfd2d6;background:#fff;padding:10px 14px;border-radius:10px;font-weight:850;color:var(--text)}.choice-btn:hover{background:#eff8f8}
.concept-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:14px}.concept-grid>div{border:1px solid var(--line);border-radius:12px;padding:15px;background:#f9fcfc}.concept-grid p{margin:.4rem 0 0;color:var(--muted);line-height:1.5;font-size:.9rem}
@media(max-width:900px){.toolbar{grid-template-columns:1fr 1fr}.toolbar-buttons{grid-column:1/-1}.table-header{flex-direction:column}.concept-grid{grid-template-columns:1fr}.series-row{overflow:auto;grid-template-columns:60px repeat(15,52px)}}
@media(max-width:600px){.toolbar{grid-template-columns:1fr}.toolbar-buttons{grid-column:auto}.element-panel{align-items:flex-start}.stat-grid{grid-template-columns:1fr 1fr}}