diff --git a/render.py b/render.py
index 7f19118..a727721 100644
--- a/render.py
+++ b/render.py
@@ -64,6 +64,15 @@ WIDE_PALETTES = {
     "hieroglyph", "cuneiform", "alchemical",
 }
 
+# Palettes whose glyphs aren't reliably monospaced in JetBrains Mono.
+# These need fixed-width cells in HTML output to maintain grid alignment.
+VARIABLE_WIDTH_PALETTES = WIDE_PALETTES | {
+    "wingdings", "zodiac", "chess", "arrows", "music", "cosmic", "mystic",
+    "runes", "geometric", "kanji", "thai", "arabic", "devanagari", "ethiopic",
+    "georgian", "tibetan", "hieroglyph", "cuneiform", "alchemical", "dingbats",
+    "yijing", "math", "flora", "weather",
+}
+
 
 def is_wide_palette(name: str) -> bool:
     """Check if a palette uses double-width (emoji/supplementary) characters."""
@@ -128,6 +137,7 @@ def _render_frame(
     double_width: bool = False,
     dither: bool = False,
     output_format: str = "ansi",
+    fixed_width_cells: bool = False,
 ) -> str:
     """Render a single prepared (resized RGB) frame to art string."""
     width, height = img.size
@@ -159,13 +169,18 @@ def _render_frame(
                 char = chars[idx]
                 if double_width:
                     char = char + " "
+                escaped = html_escape(char)
+                if fixed_width_cells:
+                    # Wrap each char in a fixed-width cell so non-monospace
+                    # glyphs don't break the grid alignment
+                    escaped = f'<span class="c">{escaped}</span>'
                 color = f"{r},{g},{b}"
                 if color == prev_color:
-                    run_chars.append(html_escape(char))
+                    run_chars.append(escaped)
                 else:
                     if run_chars:
                         line_parts.append(f'<span style="color:rgb({prev_color})">{"".join(run_chars)}</span>')
-                    run_chars = [html_escape(char)]
+                    run_chars = [escaped]
                     prev_color = color
             if run_chars:
                 line_parts.append(f'<span style="color:rgb({prev_color})">{"".join(run_chars)}</span>')
@@ -216,7 +231,8 @@ def image_to_art(
     wide = is_wide_palette(palette_name)
     img = _prepare_frame(img, width, bg, wide_chars=wide)
     chars = PALETTES.get(palette_name, PALETTES["wingdings"])
-    return _render_frame(img, chars, double_width, dither, output_format)
+    fix_cells = palette_name in VARIABLE_WIDTH_PALETTES and output_format == "html"
+    return _render_frame(img, chars, double_width, dither, output_format, fixed_width_cells=fix_cells)
 
 
 def gif_to_art(
@@ -232,6 +248,7 @@ def gif_to_art(
     img = Image.open(image_path)
     wide = is_wide_palette(palette_name)
     chars = PALETTES.get(palette_name, PALETTES["wingdings"])
+    fix_cells = palette_name in VARIABLE_WIDTH_PALETTES and output_format == "html"
 
     frames = []
     frame_list = list(ImageSequence.Iterator(img))
@@ -245,7 +262,7 @@ def gif_to_art(
     for frame in frame_list:
         duration = frame.info.get("duration", 100)
         prepared = _prepare_frame(frame.copy(), width, bg, wide_chars=wide)
-        art = _render_frame(prepared, chars, double_width, dither, output_format)
+        art = _render_frame(prepared, chars, double_width, dither, output_format, fixed_width_cells=fix_cells)
         frames.append({"art": art, "duration": max(duration, 20)})
 
     return frames
@@ -264,7 +281,8 @@ def render_from_pil(
     wide = is_wide_palette(palette_name)
     prepared = _prepare_frame(img, width, bg, wide_chars=wide)
     chars = PALETTES.get(palette_name, PALETTES["wingdings"])
-    return _render_frame(prepared, chars, double_width, dither, output_format)
+    fix_cells = palette_name in VARIABLE_WIDTH_PALETTES and output_format == "html"
+    return _render_frame(prepared, chars, double_width, dither, output_format, fixed_width_cells=fix_cells)
 
 
 def is_animated_gif(image_path: str) -> bool:
@@ -428,12 +446,13 @@ def animate_image(
     prepared = _prepare_frame(img, width, bg, wide_chars=wide)
     chars = PALETTES.get(palette_name, PALETTES["wingdings"])
     effect_fn = _EFFECTS.get(effect, _effect_color_cycle)
+    fix_cells = palette_name in VARIABLE_WIDTH_PALETTES and output_format == "html"
 
     frames = []
     for i in range(num_frames):
         t = i / num_frames
         effected = effect_fn(prepared, t)
-        art = _render_frame(effected, chars, double_width, dither, output_format)
+        art = _render_frame(effected, chars, double_width, dither, output_format, fixed_width_cells=fix_cells)
         frame = {"art": art, "duration": frame_duration}
         if return_pil:
             frame["pil_frame"] = effected
@@ -459,6 +478,7 @@ def generate_pattern(
 
     chars = PALETTES.get(palette_name, PALETTES["wingdings"])
     num_chars = len(chars)
+    fix_cells = palette_name in VARIABLE_WIDTH_PALETTES and output_format == "html"
 
     # Random parameters for variety
     freq1 = rng.uniform(0.02, 0.12)
@@ -627,7 +647,10 @@ def generate_pattern(
             char = chars[idx]
 
             if output_format == "html":
-                line.append(f'<span style="color:rgb({r},{g},{b})">{html_escape(char)}</span>')
+                escaped = html_escape(char)
+                if fix_cells:
+                    escaped = f'<span class="c">{escaped}</span>'
+                line.append(f'<span style="color:rgb({r},{g},{b})">{escaped}</span>')
             elif output_format == "ansi":
                 line.append(f"\033[38;2;{r};{g};{b}m{char}\033[0m")
             else:
diff --git a/static/app.js b/static/app.js
index 8108e45..f90f5e0 100644
--- a/static/app.js
+++ b/static/app.js
@@ -257,9 +257,40 @@ async function generatePattern() {
   randomBtn.disabled = false;
 }
 
-// ── GIF Animation ──────────────────────────────
+// ── GIF Animation (runs even when tab is inactive) ──────────────────────────────
 
 let animFrames = null;
+let animationWorker = null;
+
+function createAnimationWorker() {
+  const blob = new Blob([`
+    let timerId = null;
+    self.onmessage = function(e) {
+      if (e.data.cmd === 'start') {
+        if (timerId) clearTimeout(timerId);
+        timerId = setTimeout(() => self.postMessage('tick'), e.data.delay);
+      } else if (e.data.cmd === 'stop') {
+        if (timerId) { clearTimeout(timerId); timerId = null; }
+      }
+    };
+  `], { type: 'application/javascript' });
+  return new Worker(URL.createObjectURL(blob));
+}
+
+function scheduleNextFrame(delay) {
+  if (animationWorker) {
+    animationWorker.postMessage({ cmd: 'start', delay });
+  }
+}
+
+function advanceFrame() {
+  if (!animationPlaying || !animFrames) return;
+  animFrames[currentFrameIdx].style.display = 'none';
+  currentFrameIdx = (currentFrameIdx + 1) % animFrames.length;
+  animFrames[currentFrameIdx].style.display = 'block';
+  updateFrameCounter();
+  scheduleNextFrame(parseInt(animFrames[currentFrameIdx].dataset.duration) || 100);
+}
 
 function startAnimation(frames) {
   animFrames = frames;
@@ -271,18 +302,15 @@ function startAnimation(frames) {
   playPauseBtn.innerHTML = '&#x23F8;';
   updateFrameCounter();
 
-  function nextFrame() {
-    if (!animationPlaying) return;
-    frames[currentFrameIdx].style.display = 'none';
-    currentFrameIdx = (currentFrameIdx + 1) % frames.length;
-    frames[currentFrameIdx].style.display = 'block';
-    updateFrameCounter();
-    animationInterval = setTimeout(nextFrame, parseInt(frames[currentFrameIdx].dataset.duration) || 100);
-  }
-  animationInterval = setTimeout(nextFrame, parseInt(frames[0].dataset.duration) || 100);
+  if (animationWorker) animationWorker.terminate();
+  animationWorker = createAnimationWorker();
+  animationWorker.onmessage = advanceFrame;
+
+  scheduleNextFrame(parseInt(frames[0].dataset.duration) || 100);
 }
 
 function stopAnimation() {
+  if (animationWorker) { animationWorker.postMessage({ cmd: 'stop' }); animationWorker.terminate(); animationWorker = null; }
   if (animationInterval) { clearTimeout(animationInterval); animationInterval = null; }
   animationPlaying = false;
   animFrames = null;
@@ -298,21 +326,13 @@ playPauseBtn.addEventListener('click', () => {
   if (animationPlaying) {
     // Pause
     animationPlaying = false;
-    if (animationInterval) { clearTimeout(animationInterval); animationInterval = null; }
+    if (animationWorker) animationWorker.postMessage({ cmd: 'stop' });
     playPauseBtn.innerHTML = '&#x25B6;';
   } else {
     // Resume
     animationPlaying = true;
     playPauseBtn.innerHTML = '&#x23F8;';
-    function nextFrame() {
-      if (!animationPlaying || !animFrames) return;
-      animFrames[currentFrameIdx].style.display = 'none';
-      currentFrameIdx = (currentFrameIdx + 1) % animFrames.length;
-      animFrames[currentFrameIdx].style.display = 'block';
-      updateFrameCounter();
-      animationInterval = setTimeout(nextFrame, parseInt(animFrames[currentFrameIdx].dataset.duration) || 100);
-    }
-    animationInterval = setTimeout(nextFrame, parseInt(animFrames[currentFrameIdx].dataset.duration) || 100);
+    scheduleNextFrame(parseInt(animFrames[currentFrameIdx].dataset.duration) || 100);
   }
 });
 
diff --git a/static/style.css b/static/style.css
index 1f2b89a..13aedc0 100644
--- a/static/style.css
+++ b/static/style.css
@@ -419,6 +419,14 @@ header {
   position: relative;
 }
 
+/* Fixed-width character cells for non-monospace glyphs */
+.preview-area .c {
+  display: inline-block;
+  width: 1ch;
+  text-align: center;
+  overflow: hidden;
+}
+
 .placeholder {
   position: absolute;
   top: 50%;