diff --git a/hdyc-svelte/src/app.css b/hdyc-svelte/src/app.css
index 1632c6f..2bb3aa9 100644
--- a/hdyc-svelte/src/app.css
+++ b/hdyc-svelte/src/app.css
@@ -246,6 +246,28 @@ a {
 a:hover {
   color: var(--accent-dark);
 }
+a:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+  border-radius: 6px;
+}
+
+.skip-link {
+  position: fixed;
+  top: 0.6rem;
+  left: 0.6rem;
+  z-index: 500;
+  background: var(--bg-elevated);
+  color: var(--text);
+  border: 1px solid var(--border);
+  border-radius: 10px;
+  padding: 0.5rem 0.8rem;
+  transform: translateY(-160%);
+  transition: transform 0.2s ease;
+}
+.skip-link:focus-visible {
+  transform: translateY(0);
+}
 
 /* ─── Layout Shell ───────────────────────────────────────── */
 
@@ -264,6 +286,12 @@ a:hover {
   border-bottom: 1px solid var(--border);
 }
 
+.header-left {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+}
+
 .site-logo {
   display: flex;
   align-items: center;
@@ -281,15 +309,51 @@ a:hover {
 .header-right {
   display: flex;
   align-items: center;
-  gap: 1rem;
+  gap: 0.65rem;
+}
+
+.desktop-header-search {
+  width: min(420px, 42vw);
+}
+
+.header-icon-btn {
+  display: none;
+  align-items: center;
+  justify-content: center;
+  width: 44px;
+  height: 44px;
+  border: 1px solid var(--border);
+  border-radius: 50%;
+  background: var(--input-bg);
+  color: var(--text);
+  cursor: pointer;
+  font-size: 1rem;
+  line-height: 1;
+  transition: border-color 0.2s, box-shadow 0.2s, background 0.2s;
+}
+.header-icon-btn:hover {
+  border-color: var(--accent);
+}
+.header-icon-btn:focus-visible {
+  outline: none;
+  box-shadow: 0 0 0 3px var(--accent-glow);
+}
+
+.mobile-header-search {
+  padding: 0.75rem 1rem;
+  border-bottom: 1px solid var(--border);
+  background: var(--header-bg);
+}
+.mobile-header-search[hidden] {
+  display: none !important;
 }
 
 .theme-toggle {
   display: inline-flex;
   align-items: center;
   justify-content: center;
-  width: 36px;
-  height: 36px;
+  width: 44px;
+  height: 44px;
   border-radius: 50%;
   border: 1px solid var(--border);
   background: var(--input-bg);
@@ -395,12 +459,25 @@ a:hover {
 
 .hamburger {
   display: none;
-  background: none;
-  border: none;
+  align-items: center;
+  justify-content: center;
+  width: 44px;
+  height: 44px;
+  background: var(--input-bg);
+  border: 1px solid var(--border);
+  border-radius: 50%;
   color: var(--text);
-  font-size: 1.4rem;
+  font-size: 1.2rem;
   cursor: pointer;
-  padding: 0.25rem;
+  padding: 0;
+  transition: border-color 0.2s, box-shadow 0.2s, background 0.2s;
+}
+.hamburger:hover {
+  border-color: var(--accent);
+}
+.hamburger:focus-visible {
+  outline: none;
+  box-shadow: 0 0 0 3px var(--accent-glow);
 }
 
 .site-body {
@@ -419,6 +496,9 @@ a:hover {
   padding: clamp(1.5rem, 2vw, 3rem);
   width: 100%;
 }
+.main-content:focus {
+  outline: none;
+}
 
 .site-footer {
   padding: 2rem;
@@ -431,22 +511,35 @@ a:hover {
 /* ─── Page Utilities ─────────────────────────────────────── */
 
 .breadcrumbs {
-  display: flex;
-  align-items: center;
-  gap: 0.4rem;
   font-size: 0.82rem;
   color: var(--text-muted);
   margin-bottom: 1.5rem;
 }
+.breadcrumbs ol {
+  list-style: none;
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.4rem;
+  margin: 0;
+  padding: 0;
+}
+.breadcrumbs li {
+  display: inline-flex;
+  align-items: center;
+  color: var(--text-muted);
+}
+.breadcrumbs li + li::before {
+  content: '›';
+  opacity: 0.4;
+  margin-right: 0.4rem;
+}
 .breadcrumbs a {
   color: var(--text-muted);
 }
 .breadcrumbs a:hover {
   color: var(--accent);
 }
-.breadcrumbs .sep {
-  opacity: 0.4;
-}
 
 .page-title {
   font-size: 1.8rem;
@@ -459,6 +552,10 @@ a:hover {
   background-clip: text;
 }
 
+.calculator-page-title {
+  margin-bottom: 1.25rem;
+}
+
 .page-subtitle {
   color: var(--text-muted);
   font-size: 1rem;
@@ -531,6 +628,11 @@ a:hover {
   box-shadow: 0 4px 16px rgba(0, 0, 0, 0.15);
   color: var(--accent);
 }
+.calc-list-item:focus-visible {
+  outline: none;
+  border-color: var(--accent);
+  box-shadow: 0 0 0 3px var(--accent-glow);
+}
 
 /* ─── Related Converters ─────────────────────────────────── */
 
@@ -554,6 +656,11 @@ a:hover {
   color: var(--accent);
   border-color: var(--accent);
 }
+.related-chip:focus-visible {
+  outline: none;
+  border-color: var(--accent);
+  box-shadow: 0 0 0 3px var(--accent-glow);
+}
 
 /* ─── SEO Content ────────────────────────────────────────── */
 
@@ -633,7 +740,7 @@ a:hover {
 
 @media (max-width: 1024px) {
   .hamburger {
-    display: block;
+    display: inline-flex;
   }
   .site-body {
     gap: 1rem;
@@ -642,9 +749,31 @@ a:hover {
 }
 
 @media (max-width: 768px) {
+  .site-header {
+    padding: 0 1rem;
+  }
+  .site-logo {
+    font-size: 1rem;
+    gap: 0.4rem;
+  }
+  .desktop-header-search {
+    display: none;
+  }
+  .header-icon-btn {
+    display: inline-flex;
+  }
+  .mobile-header-search {
+    padding: 0.65rem 1rem;
+  }
   .main-content {
     padding: 1.25rem;
   }
+  .breadcrumbs {
+    margin-bottom: 1rem;
+  }
+  .page-title {
+    font-size: 1.5rem;
+  }
   .hero h1 {
     font-size: 1.8rem;
   }
@@ -665,6 +794,9 @@ a:hover {
   .stats-row {
     gap: 1.5rem;
   }
+  .seo-content {
+    padding: 1.25rem;
+  }
   .site-body {
     gap: 1rem;
     padding-inline: 1rem;
diff --git a/hdyc-svelte/src/hooks.server.ts b/hdyc-svelte/src/hooks.server.ts
index e081ac8..1c4a2cb 100644
--- a/hdyc-svelte/src/hooks.server.ts
+++ b/hdyc-svelte/src/hooks.server.ts
@@ -13,18 +13,38 @@ const MIME_TYPES: Record<string, string> = {
   '.otf': 'font/otf'
 };
 
+const HTML_CACHE_CONTROL = 'public, max-age=0, must-revalidate';
+const ASSET_404_CACHE_CONTROL = 'no-store';
+
 export const handle: Handle = async ({ event, resolve }) => {
   const response = await resolve(event);
-  if (event.url.pathname.startsWith('/_app/')) {
+  const pathname = event.url.pathname;
+  const contentType = response.headers.get('content-type') ?? '';
+
+  if (pathname.startsWith('/_app/')) {
     const existing = response.headers.get('content-type');
     const hasValidHeader = existing && existing.trim().length > 0;
     if (!hasValidHeader) {
-      const extension = path.extname(event.url.pathname).toLowerCase();
+      const extension = path.extname(pathname).toLowerCase();
       const mime = extension && MIME_TYPES[extension];
       if (mime) {
         response.headers.set('content-type', mime);
       }
     }
+
+    // Missing hashed assets should never be cached; otherwise stale HTML can
+    // keep pointing to already-rotated files long after a deployment.
+    if (response.status >= 400) {
+      response.headers.set('cache-control', ASSET_404_CACHE_CONTROL);
+    }
+
+    return response;
+  }
+
+  // HTML documents should revalidate so they can reference the latest client
+  // bundle hashes after each deployment.
+  if (contentType.includes('text/html')) {
+    response.headers.set('cache-control', HTML_CACHE_CONTROL);
   }
 
   return response;
diff --git a/hdyc-svelte/src/lib/assets/favicon.svg b/hdyc-svelte/src/lib/assets/favicon.svg
index cc5dc66..e45a622 100644
--- a/hdyc-svelte/src/lib/assets/favicon.svg
+++ b/hdyc-svelte/src/lib/assets/favicon.svg
@@ -1 +1,21 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="107" height="128" viewBox="0 0 107 128"><title>svelte-logo</title><path d="M94.157 22.819c-10.4-14.885-30.94-19.297-45.792-9.835L22.282 29.608A29.92 29.92 0 0 0 8.764 49.65a31.5 31.5 0 0 0 3.108 20.231 30 30 0 0 0-4.477 11.183 31.9 31.9 0 0 0 5.448 24.116c10.402 14.887 30.942 19.297 45.791 9.835l26.083-16.624A29.92 29.92 0 0 0 98.235 78.35a31.53 31.53 0 0 0-3.105-20.232 30 30 0 0 0 4.474-11.182 31.88 31.88 0 0 0-5.447-24.116" style="fill:#ff3e00"/><path d="M45.817 106.582a20.72 20.72 0 0 1-22.237-8.243 19.17 19.17 0 0 1-3.277-14.503 18 18 0 0 1 .624-2.435l.49-1.498 1.337.981a33.6 33.6 0 0 0 10.203 5.098l.97.294-.09.968a5.85 5.85 0 0 0 1.052 3.878 6.24 6.24 0 0 0 6.695 2.485 5.8 5.8 0 0 0 1.603-.704L69.27 76.28a5.43 5.43 0 0 0 2.45-3.631 5.8 5.8 0 0 0-.987-4.371 6.24 6.24 0 0 0-6.698-2.487 5.7 5.7 0 0 0-1.6.704l-9.953 6.345a19 19 0 0 1-5.296 2.326 20.72 20.72 0 0 1-22.237-8.243 19.17 19.17 0 0 1-3.277-14.502 17.99 17.99 0 0 1 8.13-12.052l26.081-16.623a19 19 0 0 1 5.3-2.329 20.72 20.72 0 0 1 22.237 8.243 19.17 19.17 0 0 1 3.277 14.503 18 18 0 0 1-.624 2.435l-.49 1.498-1.337-.98a33.6 33.6 0 0 0-10.203-5.1l-.97-.294.09-.968a5.86 5.86 0 0 0-1.052-3.878 6.24 6.24 0 0 0-6.696-2.485 5.8 5.8 0 0 0-1.602.704L37.73 51.72a5.42 5.42 0 0 0-2.449 3.63 5.79 5.79 0 0 0 .986 4.372 6.24 6.24 0 0 0 6.698 2.486 5.8 5.8 0 0 0 1.602-.704l9.952-6.342a19 19 0 0 1 5.295-2.328 20.72 20.72 0 0 1 22.237 8.242 19.17 19.17 0 0 1 3.277 14.503 18 18 0 0 1-8.13 12.053l-26.081 16.622a19 19 0 0 1-5.3 2.328" style="fill:#fff"/></svg>
\ No newline at end of file
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="Calculator favicon">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#115e59" />
+      <stop offset="1" stop-color="#0284c7" />
+    </linearGradient>
+  </defs>
+  <rect width="64" height="64" rx="14" fill="url(#bg)" />
+  <rect x="17" y="8" width="30" height="48" rx="6" fill="#f8fafc" stroke="#0f172a" stroke-width="2" />
+  <rect x="22" y="14" width="20" height="9" rx="2" fill="#0f172a" />
+  <rect x="24" y="17" width="16" height="3" rx="1.5" fill="#67e8f9" />
+  <g fill="#0ea5e9">
+    <rect x="22" y="28" width="6" height="6" rx="1.5" />
+    <rect x="29" y="28" width="6" height="6" rx="1.5" />
+    <rect x="36" y="28" width="6" height="6" rx="1.5" />
+    <rect x="22" y="35" width="6" height="6" rx="1.5" />
+    <rect x="29" y="35" width="6" height="6" rx="1.5" />
+    <rect x="36" y="35" width="6" height="13" rx="1.5" fill="#f97316" />
+    <rect x="22" y="42" width="13" height="6" rx="1.5" fill="#14b8a6" />
+  </g>
+</svg>
diff --git a/hdyc-svelte/src/lib/components/Calculator.svelte b/hdyc-svelte/src/lib/components/Calculator.svelte
index 74fd21e..cd2fc57 100644
--- a/hdyc-svelte/src/lib/components/Calculator.svelte
+++ b/hdyc-svelte/src/lib/components/Calculator.svelte
@@ -8,6 +8,7 @@
   import QuickConversionTable from '$lib/components/QuickConversionTable.svelte';
 
   export let config: CalculatorDef;
+  export let showTitle = true;
 
   let val1 = '';
   let val2 = '';
@@ -70,12 +71,16 @@
 </script>
 
 <div class="calculator-card">
-  <div class="calc-header">
-    <h2>{config.name}</h2>
-    {#if config.teaser}
-      <p class="calc-subtitle">{config.teaser}</p>
-    {/if}
-  </div>
+  {#if showTitle || config.teaser}
+    <div class="calc-header">
+      {#if showTitle}
+        <h2>{config.name}</h2>
+      {/if}
+      {#if config.teaser}
+        <p class="calc-subtitle" class:no-title={!showTitle}>{config.teaser}</p>
+      {/if}
+    </div>
+  {/if}
 
   <div class="calc-body" class:three-col={has3}>
     <div class="input-group">
@@ -181,6 +186,9 @@
     color: rgba(255, 255, 255, 0.85);
     font-weight: 400;
   }
+  .calc-subtitle.no-title {
+    margin-top: 0;
+  }
 
   .calc-body {
     display: grid;
@@ -261,6 +269,10 @@
     background: var(--accent-dark);
     transform: rotate(180deg);
   }
+  .swap-btn:focus-visible {
+    outline: none;
+    box-shadow: 0 0 0 3px var(--accent-glow);
+  }
 
   .calc-footer {
     display: flex;
@@ -284,16 +296,34 @@
     color: #fff;
     border-color: var(--accent);
   }
+  .clear-btn:focus-visible {
+    outline: none;
+    box-shadow: 0 0 0 3px var(--accent-glow);
+  }
   .formula-hint {
     font-size: 0.78rem;
     color: var(--text-muted);
     font-family: 'JetBrains Mono', monospace;
   }
 
+  @media (max-width: 900px) {
+    .calc-footer {
+      flex-wrap: wrap;
+      gap: 0.75rem;
+    }
+    .formula-hint {
+      width: 100%;
+    }
+  }
+
   @media (max-width: 640px) {
+    .calc-header {
+      padding: 1.2rem 1.2rem 0.9rem;
+    }
     .calc-body {
       grid-template-columns: 1fr;
       gap: 0.75rem;
+      padding: 1.25rem;
     }
     .calc-body.three-col {
       grid-template-columns: 1fr;
@@ -307,5 +337,8 @@
     .swap-btn:hover {
       transform: rotate(270deg);
     }
+    .calc-footer {
+      padding: 0.9rem 1.25rem 1rem;
+    }
   }
 </style>
diff --git a/hdyc-svelte/src/lib/components/QuickConversionExample.svelte b/hdyc-svelte/src/lib/components/QuickConversionExample.svelte
index 01ec7d8..e75ef49 100644
--- a/hdyc-svelte/src/lib/components/QuickConversionExample.svelte
+++ b/hdyc-svelte/src/lib/components/QuickConversionExample.svelte
@@ -97,4 +97,11 @@
     font-weight: 600;
     margin-left: 0.35rem;
   }
+
+  @media (max-width: 640px) {
+    .example-card {
+      margin: 0 1.25rem 1.25rem;
+      padding: 1rem;
+    }
+  }
 </style>
diff --git a/hdyc-svelte/src/lib/components/QuickConversionTable.svelte b/hdyc-svelte/src/lib/components/QuickConversionTable.svelte
index b2d0dfe..5c0245b 100644
--- a/hdyc-svelte/src/lib/components/QuickConversionTable.svelte
+++ b/hdyc-svelte/src/lib/components/QuickConversionTable.svelte
@@ -98,4 +98,17 @@
   .chart-output-unit {
     font-variant: petite-caps;
   }
+
+  @media (max-width: 640px) {
+    .quick-chart {
+      margin: 0.75rem 1.25rem 1.25rem;
+      padding: 0.9rem 1rem;
+    }
+    .chart-row {
+      font-size: 0.9rem;
+    }
+    .chart-statement {
+      line-height: 1.35;
+    }
+  }
 </style>
diff --git a/hdyc-svelte/src/lib/components/QuickDefinitionCard.svelte b/hdyc-svelte/src/lib/components/QuickDefinitionCard.svelte
index dab4d7c..c5536c1 100644
--- a/hdyc-svelte/src/lib/components/QuickDefinitionCard.svelte
+++ b/hdyc-svelte/src/lib/components/QuickDefinitionCard.svelte
@@ -68,4 +68,14 @@
     font-size: 0.85rem;
     color: var(--text-muted);
   }
+
+  @media (max-width: 640px) {
+    .definition-card {
+      margin: 0 1.25rem 1.25rem;
+      padding: 1rem;
+    }
+    .definition-grid {
+      grid-template-columns: 1fr;
+    }
+  }
 </style>
diff --git a/hdyc-svelte/src/lib/components/SearchBar.svelte b/hdyc-svelte/src/lib/components/SearchBar.svelte
index 69aff7f..40c85a1 100644
--- a/hdyc-svelte/src/lib/components/SearchBar.svelte
+++ b/hdyc-svelte/src/lib/components/SearchBar.svelte
@@ -2,11 +2,27 @@
   import { searchCalculators } from '$lib/data/calculators';
   import { goto } from '$app/navigation';
 
+  export let idPrefix = 'search';
+
   let query = '';
   let focused = false;
   let selectedIndex = -1;
+  let lastQuery = '';
 
   $: results = query.length >= 2 ? searchCalculators(query).slice(0, 8) : [];
+  $: listboxId = `${idPrefix}-listbox`;
+  $: inputId = `${idPrefix}-input`;
+  $: isOpen = focused && results.length > 0;
+  $: activeDescendant = selectedIndex >= 0 && isOpen ? `${idPrefix}-option-${selectedIndex}` : undefined;
+
+  $: if (query !== lastQuery) {
+    selectedIndex = -1;
+    lastQuery = query;
+  }
+
+  $: if (selectedIndex >= results.length) {
+    selectedIndex = results.length - 1;
+  }
 
   function handleKeydown(e: KeyboardEvent) {
     if (e.key === 'ArrowDown') {
@@ -33,12 +49,13 @@
   }
 </script>
 
-<div class="search-wrapper" class:active={focused && results.length > 0}>
+<div class="search-wrapper" class:active={isOpen}>
   <div class="search-input-wrap">
     <svg class="search-icon" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
       <circle cx="11" cy="11" r="8" /><path d="m21 21-4.35-4.35" />
     </svg>
     <input
+      id={inputId}
       type="text"
       bind:value={query}
       on:focus={() => (focused = true)}
@@ -46,6 +63,12 @@
       on:keydown={handleKeydown}
       placeholder="Search conversions..."
       aria-label="Search conversions"
+      role="combobox"
+      aria-autocomplete="list"
+      aria-haspopup="listbox"
+      aria-expanded={isOpen ? 'true' : 'false'}
+      aria-controls={listboxId}
+      aria-activedescendant={activeDescendant}
     />
     {#if query}
       <button
@@ -61,11 +84,12 @@
       </button>
     {/if}
   </div>
-  {#if focused && results.length > 0}
-    <ul class="results" role="listbox" aria-label="Conversion suggestions">
+  {#if isOpen}
+    <ul class="results" id={listboxId} role="listbox" aria-label="Conversion suggestions">
       {#each results as result, i}
         <li>
           <button
+            id={`${idPrefix}-option-${i}`}
             type="button"
             class="result-item"
             class:selected={i === selectedIndex}
@@ -162,6 +186,10 @@
   .result-item.selected {
     background: var(--hover-bg);
   }
+  .result-item:focus-visible {
+    outline: none;
+    background: var(--hover-bg);
+  }
   .result-name {
     font-weight: 500;
   }
diff --git a/hdyc-svelte/src/lib/components/Sidebar.svelte b/hdyc-svelte/src/lib/components/Sidebar.svelte
index 44147c0..360be04 100644
--- a/hdyc-svelte/src/lib/components/Sidebar.svelte
+++ b/hdyc-svelte/src/lib/components/Sidebar.svelte
@@ -131,12 +131,31 @@
   }
 
   export let open = false;
+  $: isSidebarHidden = !isDesktop && !open;
+
+  function closeSidebar() {
+    open = false;
+  }
+
+  function handleWindowKeydown(event: KeyboardEvent) {
+    if (event.key === 'Escape' && open && !isDesktop) {
+      closeSidebar();
+    }
+  }
 </script>
 
-<aside class="sidebar" class:open id="site-navigation" aria-hidden={open ? 'false' : 'true'}>
+<svelte:window on:keydown={handleWindowKeydown} />
+
+<aside
+  class="sidebar"
+  class:open={open}
+  id="site-navigation"
+  aria-hidden={isSidebarHidden ? 'true' : undefined}
+  inert={isSidebarHidden}
+>
   <div class="sidebar-header">
     <h3>All Converters</h3>
-    <button class="close-btn" on:click={() => (open = false)} aria-label="Close sidebar">✕</button>
+    <button type="button" class="close-btn" on:click={closeSidebar} aria-label="Close sidebar">✕</button>
   </div>
   <nav aria-label="Calculator categories">
     {#each categoryUnitGroups as group}
@@ -200,10 +219,8 @@
   </nav>
 </aside>
 
-{#if open}
-  <!-- svelte-ignore a11y_click_events_have_key_events -->
-  <!-- svelte-ignore a11y_no_static_element_interactions -->
-  <div class="overlay" on:click={() => (open = false)}></div>
+{#if open && !isDesktop}
+  <button type="button" class="overlay" aria-label="Close sidebar" on:click={closeSidebar}></button>
 {/if}
 
 <style>
@@ -243,6 +260,11 @@
     cursor: pointer;
     padding: 0.25rem;
   }
+  .close-btn:focus-visible {
+    outline: none;
+    box-shadow: 0 0 0 3px var(--accent-glow);
+    border-radius: 8px;
+  }
 
   nav {
     padding: 0.5rem 0;
@@ -265,6 +287,10 @@
   .cat-toggle:hover {
     background: var(--hover-bg);
   }
+  .cat-toggle:focus-visible {
+    outline: none;
+    box-shadow: inset 0 0 0 2px var(--accent-glow);
+  }
   .cat-toggle.active {
     color: var(--accent);
     font-weight: 600;
@@ -306,6 +332,11 @@
     color: var(--accent);
     background: var(--hover-bg);
   }
+  .cat-list li a:focus-visible {
+    outline: none;
+    color: var(--accent);
+    background: var(--hover-bg);
+  }
   .cat-list li a.current {
     color: var(--accent);
     font-weight: 600;
@@ -337,6 +368,10 @@
   .unit-toggle:hover {
     background: var(--hover-bg);
   }
+  .unit-toggle:focus-visible {
+    outline: none;
+    box-shadow: inset 0 0 0 2px var(--accent-glow);
+  }
   .unit-toggle.expanded {
     color: var(--accent);
     font-weight: 600;
@@ -362,9 +397,15 @@
     color: var(--accent);
     background: var(--hover-bg);
   }
+  .unit-list li a:focus-visible {
+    outline: none;
+    color: var(--accent);
+    background: var(--hover-bg);
+  }
 
   .overlay {
     display: none;
+    border: 0;
   }
 
   @media (max-width: 1024px) {
diff --git a/hdyc-svelte/src/lib/seo.ts b/hdyc-svelte/src/lib/seo.ts
new file mode 100644
index 0000000..1c26fe1
--- /dev/null
+++ b/hdyc-svelte/src/lib/seo.ts
@@ -0,0 +1,46 @@
+export const SITE_URL = 'https://howdoyouconvert.com';
+export const SITE_NAME = 'HowDoYouConvert.com';
+export const DEFAULT_ROBOTS = 'index,follow';
+export const DEFAULT_TWITTER_CARD = 'summary';
+
+type OpenGraphType = 'website' | 'article';
+
+type SeoInput = {
+  title: string;
+  description: string;
+  pathname: string;
+  type?: OpenGraphType;
+};
+
+const normalizePathname = (pathname: string): string => {
+  const pathWithSlash = pathname.startsWith('/') ? pathname : `/${pathname}`;
+  const normalized = pathWithSlash.replace(/\/{2,}/g, '/');
+  if (normalized.length > 1 && normalized.endsWith('/')) {
+    return normalized.slice(0, -1);
+  }
+  return normalized;
+};
+
+export const canonicalUrl = (pathname: string): string => `${SITE_URL}${normalizePathname(pathname)}`;
+
+export const buildSeoMeta = ({ title, description, pathname, type = 'website' }: SeoInput) => {
+  const canonical = canonicalUrl(pathname);
+  return {
+    canonical,
+    robots: DEFAULT_ROBOTS,
+    og: {
+      type,
+      title,
+      description,
+      url: canonical,
+      siteName: SITE_NAME,
+    },
+    twitter: {
+      card: DEFAULT_TWITTER_CARD,
+      title,
+      description,
+    },
+  };
+};
+
+export const toJsonLd = (value: unknown): string => JSON.stringify(value).replace(/</g, '\\u003c');
diff --git a/hdyc-svelte/src/routes/+layout.svelte b/hdyc-svelte/src/routes/+layout.svelte
index ab906a6..de01751 100644
--- a/hdyc-svelte/src/routes/+layout.svelte
+++ b/hdyc-svelte/src/routes/+layout.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
   import { browser } from '$app/environment';
+  import { afterNavigate } from '$app/navigation';
   import { onMount } from 'svelte';
   import { page } from '$app/stores';
 
@@ -253,11 +254,17 @@
   ];
 
   let sidebarOpen = false;
+  let headerSearchOpen = false;
+  let isMobileHeader = false;
   let theme: ThemeMode = 'dark';
   let selectedPaletteIndex = 0;
   $: isHomepage = $page.url.pathname === '/';
-  $: if (isHomepage && sidebarOpen) {
+  $: if (isHomepage && (sidebarOpen || headerSearchOpen)) {
     sidebarOpen = false;
+    headerSearchOpen = false;
+  }
+  $: if (!isMobileHeader && headerSearchOpen) {
+    headerSearchOpen = false;
   }
 
   const applyPalette = (index: number, persist = false) => {
@@ -289,6 +296,18 @@
     applyPalette(index, true);
   };
 
+  const toggleHeaderSearch = () => {
+    headerSearchOpen = !headerSearchOpen;
+    if (headerSearchOpen) {
+      sidebarOpen = false;
+    }
+  };
+
+  afterNavigate(() => {
+    sidebarOpen = false;
+    headerSearchOpen = false;
+  });
+
   onMount(() => {
     if (!browser) return;
 
@@ -313,10 +332,30 @@
         sidebarOpen = false;
       }
     };
+    const headerBreakpoint = window.matchMedia('(max-width: 768px)');
+    const updateHeaderBreakpoint = (event?: MediaQueryListEvent) => {
+      const isCompact = event?.matches ?? headerBreakpoint.matches;
+      isMobileHeader = isCompact;
+      if (!isCompact) {
+        headerSearchOpen = false;
+      }
+    };
+    const handleHeaderBreakpoint = (event: MediaQueryListEvent) => {
+      updateHeaderBreakpoint(event);
+    };
+
+    const handleEscape = (event: KeyboardEvent) => {
+      if (event.key === 'Escape') {
+        sidebarOpen = false;
+        headerSearchOpen = false;
+      }
+    };
+    window.addEventListener('keydown', handleEscape);
 
     if (navBreakpoint.matches) {
       sidebarOpen = false;
     }
+    updateHeaderBreakpoint();
 
     const cleanup = () => {
       if ('removeEventListener' in mediaQuery) {
@@ -329,6 +368,12 @@
       } else {
         navBreakpoint.removeListener(handleNavBreakpoint);
       }
+      if ('removeEventListener' in headerBreakpoint) {
+        headerBreakpoint.removeEventListener('change', handleHeaderBreakpoint);
+      } else {
+        headerBreakpoint.removeListener(handleHeaderBreakpoint);
+      }
+      window.removeEventListener('keydown', handleEscape);
     };
 
     if ('addEventListener' in mediaQuery) {
@@ -342,6 +387,11 @@
     } else {
       navBreakpoint.addListener(handleNavBreakpoint);
     }
+    if ('addEventListener' in headerBreakpoint) {
+      headerBreakpoint.addEventListener('change', handleHeaderBreakpoint);
+    } else {
+      headerBreakpoint.addListener(handleHeaderBreakpoint);
+    }
 
     return cleanup;
   });
@@ -349,7 +399,7 @@
 
 <svelte:head>
   <meta name="viewport" content="width=device-width, initial-scale=1" />
-  <link rel="icon" href="/favicon.png" />
+  <link rel="icon" type="image/svg+xml" href="/favicon.svg?v=2" />
   <!-- Matomo Tag Manager -->
   <script>
     var _mtm = window._mtm = window._mtm || [];
@@ -362,13 +412,20 @@
   <!-- End Matomo Tag Manager -->
 </svelte:head>
 
+<a href="#main-content" class="skip-link">Skip to main content</a>
+
 <header class="site-header">
-  <div style="display:flex;align-items:center;gap:0.75rem;">
+  <div class="header-left">
     {#if !isHomepage}
       <button
         type="button"
         class="hamburger"
-        on:click={() => (sidebarOpen = !sidebarOpen)}
+        on:click={() => {
+          sidebarOpen = !sidebarOpen;
+          if (sidebarOpen) {
+            headerSearchOpen = false;
+          }
+        }}
         aria-label="Toggle menu"
         aria-controls="site-navigation"
         aria-expanded={sidebarOpen ? 'true' : 'false'}
@@ -381,15 +438,35 @@
     </a>
   </div>
   <div class="header-right">
-    <SearchBar />
+    {#if !isHomepage}
+      <div class="desktop-header-search">
+        <SearchBar idPrefix="header-search" />
+      </div>
+      <button
+        type="button"
+        class="header-icon-btn search-toggle"
+        on:click={toggleHeaderSearch}
+        aria-controls="mobile-header-search"
+        aria-expanded={headerSearchOpen ? 'true' : 'false'}
+        aria-label={headerSearchOpen ? 'Close search' : 'Open search'}
+      >
+        <span aria-hidden="true">🔍</span>
+      </button>
+    {/if}
   </div>
 </header>
 
+{#if !isHomepage}
+  <div id="mobile-header-search" class="mobile-header-search" class:open={headerSearchOpen} hidden={!headerSearchOpen}>
+    <SearchBar idPrefix="mobile-header-search" />
+  </div>
+{/if}
+
 <div class="site-body">
   {#if !isHomepage}
     <Sidebar bind:open={sidebarOpen} />
   {/if}
-  <main class="main-content">
+  <main id="main-content" class="main-content" tabindex="-1">
     <slot />
   </main>
 </div>
diff --git a/hdyc-svelte/src/routes/+page.svelte b/hdyc-svelte/src/routes/+page.svelte
index 7155200..6f836e1 100644
--- a/hdyc-svelte/src/routes/+page.svelte
+++ b/hdyc-svelte/src/routes/+page.svelte
@@ -2,6 +2,7 @@
   import { categories, calculators } from '$lib/data/calculators';
   import CategoryCard from '$lib/components/CategoryCard.svelte';
   import SearchBar from '$lib/components/SearchBar.svelte';
+  import { buildSeoMeta, SITE_NAME, SITE_URL, toJsonLd } from '$lib/seo';
 
   const requiredCategoryFallbacks: Record<string, { label: string; icon: string }> = {
     fluids: { label: 'Fluids', icon: '💧' },
@@ -19,18 +20,43 @@
   }));
   const totalCalculators = calculators.length;
   const totalCategories = Object.keys(homepageCategories).length;
+  const pageTitle = `${SITE_NAME} — Free Unit Conversion Calculators`;
+  const pageDescription = 'Convert between hundreds of units instantly. Free online calculators for length, weight, temperature, volume, area, speed, energy, power, data and more.';
+  const seo = buildSeoMeta({
+    title: pageTitle,
+    description: pageDescription,
+    pathname: '/',
+  });
+  const websiteJsonLd = toJsonLd({
+    '@context': 'https://schema.org',
+    '@type': 'WebSite',
+    name: SITE_NAME,
+    description: pageDescription,
+    url: SITE_URL,
+  });
 </script>
 
 <svelte:head>
-  <title>HowDoYouConvert.com — Free Unit Conversion Calculators</title>
-  <meta name="description" content="Convert between hundreds of units instantly. Free online calculators for length, weight, temperature, volume, area, speed, energy, power, data and more." />
+  <title>{pageTitle}</title>
+  <meta name="description" content={pageDescription} />
+  <meta name="robots" content={seo.robots} />
+  <link rel="canonical" href={seo.canonical} />
+  <meta property="og:type" content={seo.og.type} />
+  <meta property="og:title" content={seo.og.title} />
+  <meta property="og:description" content={seo.og.description} />
+  <meta property="og:url" content={seo.og.url} />
+  <meta property="og:site_name" content={seo.og.siteName} />
+  <meta name="twitter:card" content={seo.twitter.card} />
+  <meta name="twitter:title" content={seo.twitter.title} />
+  <meta name="twitter:description" content={seo.twitter.description} />
+  <script type="application/ld+json">{websiteJsonLd}</script>
 </svelte:head>
 
 <section class="hero">
   <h1>How Do You Convert?</h1>
   <p>Fast, bidirectional unit conversions with no ads.</p>
   <div class="search-center">
-    <SearchBar />
+    <SearchBar idPrefix="home-search" />
   </div>
 </section>
 
diff --git a/hdyc-svelte/src/routes/[slug]/+page.svelte b/hdyc-svelte/src/routes/[slug]/+page.svelte
index 158d1b1..c74055a 100644
--- a/hdyc-svelte/src/routes/[slug]/+page.svelte
+++ b/hdyc-svelte/src/routes/[slug]/+page.svelte
@@ -4,35 +4,95 @@
   import { browser } from '$app/environment';
   import { onMount } from 'svelte';
   import type { PageData } from './$types';
+  import { buildSeoMeta, canonicalUrl, SITE_NAME, SITE_URL, toJsonLd } from '$lib/seo';
 
   export let data: PageData;
 
   $: calc = data.calculator;
   $: related = data.related;
+  $: pageTitle = `${calc.name} — ${SITE_NAME}`;
+  $: pageDescription = `Convert ${calc.labels.in1} to ${calc.labels.in2} instantly with our free online calculator. Accurate bidirectional conversion with the exact formula shown.`;
+  $: seo = buildSeoMeta({
+    title: pageTitle,
+    description: pageDescription,
+    pathname: `/${calc.slug}`,
+  });
+  $: breadcrumbJsonLd = toJsonLd({
+    '@context': 'https://schema.org',
+    '@type': 'BreadcrumbList',
+    itemListElement: [
+      {
+        '@type': 'ListItem',
+        position: 1,
+        name: 'Home',
+        item: SITE_URL,
+      },
+      {
+        '@type': 'ListItem',
+        position: 2,
+        name: data.categoryLabel,
+        item: canonicalUrl(`/category/${calc.category}`),
+      },
+      {
+        '@type': 'ListItem',
+        position: 3,
+        name: calc.name,
+        item: seo.canonical,
+      },
+    ],
+  });
+  $: webPageJsonLd = toJsonLd({
+    '@context': 'https://schema.org',
+    '@type': 'WebPage',
+    name: calc.name,
+    description: pageDescription,
+    url: seo.canonical,
+    isPartOf: {
+      '@type': 'WebSite',
+      name: SITE_NAME,
+      url: SITE_URL,
+    },
+  });
+
+  afterNavigate(() => {
+    if (!browser) return;
+    window.scrollTo({ top: 0 });
+  });
 
   onMount(() => {
     if (!browser) return;
     window.scrollTo({ top: 0 });
-    return afterNavigate(() => {
-      window.scrollTo({ top: 0 });
-    });
   });
 </script>
 
 <svelte:head>
-  <title>{calc.name} — HowDoYouConvert.com</title>
-  <meta name="description" content="Convert {calc.labels.in1} to {calc.labels.in2} instantly with our free online calculator. Accurate bidirectional conversion with the exact formula shown." />
+  <title>{pageTitle}</title>
+  <meta name="description" content={pageDescription} />
+  <meta name="robots" content={seo.robots} />
+  <link rel="canonical" href={seo.canonical} />
+  <meta property="og:type" content={seo.og.type} />
+  <meta property="og:title" content={seo.og.title} />
+  <meta property="og:description" content={seo.og.description} />
+  <meta property="og:url" content={seo.og.url} />
+  <meta property="og:site_name" content={seo.og.siteName} />
+  <meta name="twitter:card" content={seo.twitter.card} />
+  <meta name="twitter:title" content={seo.twitter.title} />
+  <meta name="twitter:description" content={seo.twitter.description} />
+  <script type="application/ld+json">{breadcrumbJsonLd}</script>
+  <script type="application/ld+json">{webPageJsonLd}</script>
 </svelte:head>
 
-<nav class="breadcrumbs">
-  <a href="/">Home</a>
-  <span class="sep">›</span>
-  <a href="/category/{calc.category}">{data.categoryIcon} {data.categoryLabel}</a>
-  <span class="sep">›</span>
-  <span>{calc.name}</span>
+<nav class="breadcrumbs" aria-label="Breadcrumb">
+  <ol>
+    <li><a href="/">Home</a></li>
+    <li><a href="/category/{calc.category}">{data.categoryIcon} {data.categoryLabel}</a></li>
+    <li aria-current="page">{calc.name}</li>
+  </ol>
 </nav>
 
-<Calculator config={calc} />
+<h1 class="page-title calculator-page-title">{calc.name}</h1>
+
+<Calculator config={calc} showTitle={false} />
 
 <div class="seo-content">
   {#if calc.descriptionHTML}
diff --git a/hdyc-svelte/src/routes/category/[category]/+page.svelte b/hdyc-svelte/src/routes/category/[category]/+page.svelte
index f5ee8bc..cad29c6 100644
--- a/hdyc-svelte/src/routes/category/[category]/+page.svelte
+++ b/hdyc-svelte/src/routes/category/[category]/+page.svelte
@@ -1,18 +1,80 @@
 <script lang="ts">
   import type { PageData } from './$types';
+  import { buildSeoMeta, canonicalUrl, SITE_NAME, SITE_URL, toJsonLd } from '$lib/seo';
 
   export let data: PageData;
+
+  $: pageTitle = `${data.label} Converters — ${SITE_NAME}`;
+  $: pageDescription = `Browse all ${data.label.toLowerCase()} unit converters. Free online calculators for converting between ${data.label.toLowerCase()} units.`;
+  $: categoryPath = `/category/${data.category}`;
+  $: seo = buildSeoMeta({
+    title: pageTitle,
+    description: pageDescription,
+    pathname: categoryPath,
+  });
+  $: breadcrumbJsonLd = toJsonLd({
+    '@context': 'https://schema.org',
+    '@type': 'BreadcrumbList',
+    itemListElement: [
+      {
+        '@type': 'ListItem',
+        position: 1,
+        name: 'Home',
+        item: SITE_URL,
+      },
+      {
+        '@type': 'ListItem',
+        position: 2,
+        name: `${data.label} Converters`,
+        item: seo.canonical,
+      },
+    ],
+  });
+  $: collectionJsonLd = toJsonLd({
+    '@context': 'https://schema.org',
+    '@type': 'CollectionPage',
+    name: `${data.label} Converters`,
+    description: pageDescription,
+    url: seo.canonical,
+    isPartOf: {
+      '@type': 'WebSite',
+      name: SITE_NAME,
+      url: SITE_URL,
+    },
+    mainEntity: {
+      '@type': 'ItemList',
+      itemListElement: data.calculators.map((calc, index) => ({
+        '@type': 'ListItem',
+        position: index + 1,
+        name: calc.name,
+        url: canonicalUrl(`/${calc.slug}`),
+      })),
+    },
+  });
 </script>
 
 <svelte:head>
-  <title>{data.label} Converters — HowDoYouConvert.com</title>
-  <meta name="description" content="Browse all {data.label.toLowerCase()} unit converters. Free online calculators for converting between {data.label.toLowerCase()} units." />
+  <title>{pageTitle}</title>
+  <meta name="description" content={pageDescription} />
+  <meta name="robots" content={seo.robots} />
+  <link rel="canonical" href={seo.canonical} />
+  <meta property="og:type" content={seo.og.type} />
+  <meta property="og:title" content={seo.og.title} />
+  <meta property="og:description" content={seo.og.description} />
+  <meta property="og:url" content={seo.og.url} />
+  <meta property="og:site_name" content={seo.og.siteName} />
+  <meta name="twitter:card" content={seo.twitter.card} />
+  <meta name="twitter:title" content={seo.twitter.title} />
+  <meta name="twitter:description" content={seo.twitter.description} />
+  <script type="application/ld+json">{breadcrumbJsonLd}</script>
+  <script type="application/ld+json">{collectionJsonLd}</script>
 </svelte:head>
 
-<nav class="breadcrumbs">
-  <a href="/">Home</a>
-  <span class="sep">›</span>
-  <span>{data.icon} {data.label}</span>
+<nav class="breadcrumbs" aria-label="Breadcrumb">
+  <ol>
+    <li><a href="/">Home</a></li>
+    <li aria-current="page">{data.icon} {data.label}</li>
+  </ol>
 </nav>
 
 <h1 class="page-title">{data.icon} {data.label} Converters</h1>
diff --git a/hdyc-svelte/static/favicon.svg b/hdyc-svelte/static/favicon.svg
new file mode 100644
index 0000000..e45a622
--- /dev/null
+++ b/hdyc-svelte/static/favicon.svg
@@ -0,0 +1,21 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="Calculator favicon">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#115e59" />
+      <stop offset="1" stop-color="#0284c7" />
+    </linearGradient>
+  </defs>
+  <rect width="64" height="64" rx="14" fill="url(#bg)" />
+  <rect x="17" y="8" width="30" height="48" rx="6" fill="#f8fafc" stroke="#0f172a" stroke-width="2" />
+  <rect x="22" y="14" width="20" height="9" rx="2" fill="#0f172a" />
+  <rect x="24" y="17" width="16" height="3" rx="1.5" fill="#67e8f9" />
+  <g fill="#0ea5e9">
+    <rect x="22" y="28" width="6" height="6" rx="1.5" />
+    <rect x="29" y="28" width="6" height="6" rx="1.5" />
+    <rect x="36" y="28" width="6" height="6" rx="1.5" />
+    <rect x="22" y="35" width="6" height="6" rx="1.5" />
+    <rect x="29" y="35" width="6" height="6" rx="1.5" />
+    <rect x="36" y="35" width="6" height="13" rx="1.5" fill="#f97316" />
+    <rect x="22" y="42" width="13" height="6" rx="1.5" fill="#14b8a6" />
+  </g>
+</svg>
diff --git a/hdyc-svelte/static/robots.txt b/hdyc-svelte/static/robots.txt
index b6dd667..99801e1 100644
--- a/hdyc-svelte/static/robots.txt
+++ b/hdyc-svelte/static/robots.txt
@@ -1,3 +1,4 @@
 # allow crawling everything by default
 User-agent: *
 Disallow:
+Sitemap: https://howdoyouconvert.com/sitemap.xml