:root{font-family:Arial,Helvetica,sans-serif;color:#243746;background:#f4f7fa;font-size:16px;line-height:1.55;--blue:#1f4e79;--border:#cbd6df;--muted:#526575;--good:#176342;--bad:#a82f35}
*{box-sizing:border-box}body{margin:0}button,input,select{font:inherit}button,.button{border-radius:5px;padding:.65rem 1rem;min-height:44px;cursor:pointer;line-height:1.3;text-align:center;text-decoration:none;display:inline-block;font-weight:600}button:disabled{cursor:default;opacity:.6}button:focus-visible,a:focus-visible,input:focus-visible,select:focus-visible,.button:focus-within{outline:3px solid #bf7700;outline-offset:3px}a{color:var(--blue);text-underline-offset:3px}p{margin:.5rem 0 1rem}h1,h2,h3{color:#193e60;line-height:1.2;margin:0 0 1rem}h1{font-size:2.4rem;letter-spacing:-.025em}h2{font-size:1.6rem}h3{font-size:1.2rem}.site-header{background:#fff;border-bottom:1px solid var(--border)}.header-inner{max-width:1260px;margin:auto;padding:1rem 2rem;display:flex;align-items:center;justify-content:space-between;gap:1rem}.site-name{font-size:1.15rem;font-weight:700;color:#273d4d;text-decoration:none}.site-header nav{display:flex;gap:1.5rem;font-size:.94rem}.page{max-width:1260px;margin:auto;padding:2.25rem 2rem}.page-heading{display:flex;align-items:flex-start;justify-content:space-between;gap:1rem;margin-bottom:1.65rem}.eyebrow{color:#547081;font-size:.86rem;font-weight:700;letter-spacing:.07em;text-transform:uppercase;margin:0 0 .4rem}.lead{max-width:760px;margin-bottom:0;color:var(--muted)}.primary{background:var(--blue);color:white;border:1px solid var(--blue)}.primary:hover{background:#173f64}.secondary{background:#fff;color:var(--blue);border:1px solid #97afc2}.secondary:hover{background:#edf3f8}.quiet{border:1px solid transparent;background:transparent;color:var(--blue)}.quiet:hover{text-decoration:underline}.danger{color:var(--bad)}.workspace{display:grid;grid-template-columns:265px minmax(0,1fr);align-items:start;gap:2rem}.topic-panel{position:sticky;top:1rem;padding:1.1rem 0}.topic-panel label{display:block;font-weight:700;margin-bottom:.5rem}.topic-panel select{width:100%;padding:.7rem .5rem;background:white;border:1px solid #97afc2;border-radius:4px;color:#203d54;min-height:44px}.topic-panel p{font-size:.94rem}.topic-panel #topic-description{margin:1rem 0 1.6rem}.topic-panel #level-description{margin:.8rem 0 1.5rem}.topic-progress{border-top:1px solid var(--border);padding-top:1.15rem;font-size:.94rem}.help-note{margin-top:1.25rem;color:var(--muted)}.exercise{background:white;border:1px solid var(--border);border-top:4px solid var(--blue);border-radius:4px;padding:1.8rem 2rem;min-width:0}.exercise-heading{display:flex;align-items:center;justify-content:space-between;gap:1rem;margin-bottom:1.5rem}.exercise-heading .eyebrow{margin:0}.exercise h2{max-width:680px}.exercise p{max-width:760px}.table-scroll{overflow-x:auto;margin:1.4rem 0;max-width:100%}table{border-collapse:collapse;width:100%;font-size:.95rem}caption{text-align:left;font-weight:700;margin-bottom:.6rem;color:#213f57}th{text-align:left;background:#edf3f8;color:#233e53;font-weight:700}th,td{border-bottom:1px solid #d5dfe7;padding:.7rem .8rem;vertical-align:top;min-width:90px}td:not(:first-child){font-variant-numeric:tabular-nums}.notes{padding-left:1.25rem;margin:1rem 0 1.7rem;color:#475d6e;font-size:.94rem}.notes li{margin:.4rem 0}fieldset{border:0;padding:0;margin:1.7rem 0 1rem}legend{font-size:1.12rem;font-weight:700;padding:0;margin-bottom:1rem;color:#193e60}.answer-row{margin:0 0 1.1rem}.answer-row>label{display:block;margin-bottom:.4rem;font-weight:600}.answer-entry{display:flex;align-items:center;gap:.75rem}.answer-entry input,.answer-entry select{width:min(100%,290px);min-height:44px;padding:.65rem .75rem;border:1px solid #90a5b5;border-radius:4px;background:#fff;color:#213747}.answer-entry select{max-width:100%;width:380px}.answer-entry input[aria-invalid=true],.answer-entry select[aria-invalid=true]{border:2px solid var(--bad)}.field-unit{color:var(--muted);font-size:.9rem;flex-shrink:0}.field-feedback{font-size:.9rem;margin:.3rem 0 0;min-height:0}.field-feedback.correct{color:var(--good)}.field-feedback.wrong{color:var(--bad)}.entry-note{font-size:.88rem;color:var(--muted)}.answer-actions{display:flex;gap:.7rem;align-items:center;flex-wrap:wrap;margin:1.3rem 0 .6rem}.feedback{padding:1rem 1.1rem;border-left:4px solid #a47524;background:#fff8ec;margin:1.5rem 0 0}.feedback.success{border-color:var(--good);background:#eef8f2}.hints{font-size:.96rem}.hint{margin:1rem 0 0;padding:.9rem 1rem;border-left:3px solid #8ba9c0;background:#f3f7fa}.hint strong{display:block;margin-bottom:.25rem}.solution{margin-top:1.8rem;border-top:1px solid var(--border);padding-top:1.6rem}.solution-step{margin:1rem 0;padding-bottom:1rem;border-bottom:1px solid #e1e7eb}.solution-step h4{margin:0 0 .45rem;font-size:1rem}.formula{overflow-wrap:anywhere;color:#475d6e;margin:0 0 .4rem;font-variant-numeric:tabular-nums}.result{font-weight:700;color:#193e60}.progress-panel{background:#fff;border:1px solid var(--border);border-radius:4px;margin-bottom:1.8rem;padding:1.5rem}.section-heading{display:flex;justify-content:space-between;align-items:center;gap:1rem}.section-heading h2{margin:0}.progress-panel>.section-heading+p{color:var(--muted);margin-top:1rem}.progress-summary{display:flex;gap:2rem;flex-wrap:wrap;border-top:1px solid var(--border);padding-top:1rem}.stat strong{font-size:1.55rem;display:block;color:var(--blue)}.stat span{font-size:.9rem}.progress-actions{display:flex;gap:.7rem;flex-wrap:wrap;margin-top:1.5rem}.history-row{display:flex;align-items:center;justify-content:space-between;gap:1rem;padding:.8rem 0;border-bottom:1px solid #e4e9ed}.history-row p{margin:0}.history-row small{display:block;color:var(--muted);font-size:.85rem}.notice{border:1px solid #ccab68;background:#fff8e9;color:#604415;padding:1rem}.practice-footer{display:flex;justify-content:space-between;gap:1rem;margin-top:2rem;padding-top:1rem;border-top:1px solid var(--border);font-size:.85rem;color:var(--muted)}dialog{max-width:520px;border:1px solid var(--border);border-radius:6px;padding:1.8rem;color:#243746}dialog::backdrop{background:#142c4480}dialog form{display:flex;justify-content:flex-end;gap:.7rem}.skip{position:absolute;left:1rem;top:-5rem;background:white;padding:.5rem 1rem;z-index:3}.skip:focus{top:1rem}.visually-hidden{position:absolute;width:1px;height:1px;padding:0;margin:-1px;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}[hidden]{display:none!important}
@media(max-width:900px){.workspace{grid-template-columns:220px minmax(0,1fr);gap:1.25rem}.exercise{padding:1.4rem}.page{padding:1.5rem}.header-inner{padding:1rem 1.5rem}}
@media(max-width:700px){.page{padding:1.25rem 1rem}.header-inner{padding:1rem;align-items:flex-start}.site-header nav{flex-direction:column;gap:.35rem;font-size:.88rem}.page-heading{flex-wrap:wrap}.page-heading>button{margin-top:.25rem}h1{font-size:2rem}.workspace{display:block}.topic-panel{position:static;display:grid;grid-template-columns:1fr;gap:0;padding:.2rem 0 1.25rem}.topic-panel #topic-description{margin:.6rem 0 1rem}.topic-panel #level-description{margin:.6rem 0}.topic-progress{padding-top:.65rem;margin-top:.25rem}.help-note{display:none}.exercise{padding:1.25rem 1rem}.exercise-heading{gap:.5rem}.exercise-heading .eyebrow{font-size:.8rem}h2{font-size:1.35rem}.answer-entry{flex-wrap:wrap;gap:.3rem}.answer-entry input{width:100%}.answer-entry select{width:100%}.answer-actions{align-items:stretch}.answer-actions button{flex-grow:1}.practice-footer{display:block}.progress-panel{padding:1rem}.progress-summary{gap:1rem}th,td{padding:.6rem;font-size:.9rem}.history-row{align-items:flex-start}.history-row button{padding:.5rem}dialog{max-width:calc(100% - 2rem)}}
@media print{.site-header,.topic-panel,.page-heading>button,.exercise-heading>button,.answer-actions,.progress-panel,.practice-footer{display:none}.workspace{display:block}.exercise{border:0;padding:0}.page{padding:0}.answer-entry input,.answer-entry select{border:0;border-bottom:1px solid #777}.solution{break-before:auto}}

/* Practice lab: clear difficulty colours and visible progress. */
body{--level-accent:#087b83;--level-pale:#e5f7f5;--level-deep:#075961;background:radial-gradient(circle at 8% 2%,#e7f2fb 0,transparent 28%),radial-gradient(circle at 95% 25%,#f0eafb 0,transparent 30%),#f4f7fb}
body[data-level="2"]{--level-accent:#ac6214;--level-pale:#fff2dd;--level-deep:#78400b}
body[data-level="3"]{--level-accent:#7542ad;--level-pale:#f3ebff;--level-deep:#532782}
.site-header{box-shadow:0 2px 12px #183b5910}
.page-heading{position:relative;overflow:hidden;border-radius:18px;padding:2rem 2.1rem;background:linear-gradient(115deg,#173959 5%,#165e77 95%);color:white;box-shadow:0 14px 32px #193f642b}
.page-heading::after{content:"";position:absolute;right:-60px;bottom:-140px;width:340px;height:340px;border:2px solid #ffffff24;border-radius:50%;box-shadow:0 0 0 42px #ffffff0d,0 0 0 88px #ffffff0a;pointer-events:none}
.page-heading>*{position:relative;z-index:1}
.page-heading h1{color:white;font-size:2.6rem;margin-bottom:.6rem}
.page-heading .eyebrow{color:#b9e8ee}
.page-heading .lead{color:#e1eef3}
.page-heading .beta-note{max-width:760px;margin:.75rem 0 0;color:#e1eef3;font-size:.94rem}
.page-heading .beta-note a{color:#fff;font-weight:700}
.page-heading>.secondary{background:#fff;color:#17465e;border-color:#fff;box-shadow:0 4px 14px #102e4240}
.page-heading>.secondary:hover{background:#e7f7f8}
.workspace{gap:1.4rem}
.topic-panel{background:white;border:1px solid #d4e0e8;border-radius:16px;box-shadow:0 8px 24px #28496712;padding:1.15rem;top:1.1rem}
.topic-panel #topic-description{margin:.75rem 0 1.35rem}
.topic-panel #level-description{margin:.6rem 0 1.3rem}
.level-trail{display:grid;gap:.4rem;margin:.8rem 0 .7rem}
.level-step{position:relative;display:flex;flex-direction:column;justify-content:center;align-items:center;min-width:0;min-height:71px;padding:.5rem .2rem;border:2px solid #d9e5ea;border-radius:12px;background:#f9fcfd;color:#526575;box-shadow:none;transition:transform .16s ease,box-shadow .16s ease,border-color .16s ease}
.level-step:hover{transform:translateY(-2px);box-shadow:0 5px 12px #2549651c}
.level-step strong{font-size:1.12rem;line-height:1.1}
.level-step span{font-size:.69rem;font-weight:700;letter-spacing:.025em;white-space:nowrap}
.level-step[data-level="1"]{--step-accent:#087b83;--step-pale:#e5f7f5}
.level-step[data-level="2"]{--step-accent:#ac6214;--step-pale:#fff2dd}
.level-step[data-level="3"]{--step-accent:#7542ad;--step-pale:#f3ebff}
.level-step.selected{color:var(--step-accent);border-color:var(--step-accent);background:var(--step-pale);box-shadow:0 4px 12px #304d641a}
.level-step.completed::after{content:"✓";position:absolute;right:5px;top:3px;font-size:.73rem;font-weight:800;color:var(--step-accent)}
.level-step.solo::after{content:"★";font-size:.77rem}
.topic-progress{border-top:1px solid #dfe8ee}
.exercise{border:1px solid #d4e0e8;border-top:6px solid var(--level-accent);border-radius:18px;box-shadow:0 12px 34px #25496513;padding:2rem 2.2rem}
.exercise-heading{margin-bottom:1.3rem}
.exercise-heading .eyebrow{display:inline-flex;align-items:center;border-radius:999px;padding:.45rem .8rem;background:var(--level-pale);color:var(--level-deep);font-size:.78rem;letter-spacing:.035em}
.exercise h2{color:#173d5d;font-size:1.75rem}
.exercise caption{color:var(--level-deep)}
.exercise th{background:var(--level-pale);color:#234054;border-bottom:2px solid #d3e2e9}
.exercise tr:nth-child(even) td{background:#fbfdfe}
.exercise fieldset{background:#f7fafc;border:1px solid #e2ebf1;border-radius:14px;padding:1.2rem 1.3rem;margin-top:1.8rem}
.exercise legend{padding:0 .35rem;font-size:1.22rem}
.answer-row{border-bottom:1px solid #e1e9ef;padding:.2rem 0 1rem}
.answer-row:last-child{border:0;margin-bottom:0;padding-bottom:0}
.answer-entry input:focus,.answer-entry select:focus{border-color:var(--level-accent)}
.field-feedback.correct::before{content:"✓ ";font-weight:800}
.field-feedback.wrong::before{content:"↗ ";font-weight:800}
.feedback{display:flex;align-items:flex-start;gap:.85rem;border-radius:12px;border-left-width:5px;box-shadow:0 4px 14px #26456214}
.feedback::before{content:"•";display:grid;place-items:center;flex:0 0 26px;height:26px;border-radius:50%;background:#e7d5ad;color:#744d11;font-weight:800}
.feedback.success{border-color:#188253;background:#ecf9f2;color:#165538;animation:answer-celebrate .4s ease-out}
.feedback.success::before{content:"✓";background:#188253;color:white}
.feedback.try-again{border-color:#cf8a27;background:#fff8e9;color:#5e431d}
.feedback.try-again::before{content:"↗";background:#e9be73;color:#52340b}
@keyframes answer-celebrate{0%{transform:scale(.985);box-shadow:0 0 0 0 #18825338}60%{box-shadow:0 0 0 12px #18825300}100%{transform:scale(1);box-shadow:0 4px 14px #26456214}}
.progress-panel{border-radius:16px;box-shadow:0 9px 25px #25496516;padding:1.7rem}
.progress-summary{gap:.8rem;border:0;padding-top:.4rem}
.progress-summary .stat{flex:1 1 145px;min-width:145px;background:linear-gradient(145deg,#f0f7fc,#e8f3f7);border:1px solid #d6e8ee;border-radius:12px;padding:.8rem 1rem}
.progress-summary .stat strong{font-size:1.7rem}
.progress-summary .stat span{display:block;line-height:1.25}
@media(max-width:700px){.page-heading{padding:1.5rem;border-radius:14px}.page-heading h1{font-size:2rem}.page-heading>.secondary{width:100%}.topic-panel{position:static;padding:1rem;margin-bottom:1rem}.exercise{border-radius:14px;padding:1.35rem 1rem}.exercise fieldset{padding:1rem .8rem}.exercise h2{font-size:1.5rem}.level-step{min-height:62px}.progress-panel{padding:1rem}}
@media(prefers-reduced-motion:reduce){.feedback.success{animation:none}.level-step{transition:none}.level-step:hover{transform:none}}
.solution-detail{margin:.4rem 0;color:#334b5d;line-height:1.55}
.solution-explanation{max-width:760px;line-height:1.55}