diff --git a/wasm/app/app.js b/wasm/app/app.js
index 2c61f8a..1f9d71e 100644
--- a/wasm/app/app.js
+++ b/wasm/app/app.js
@@ -263,7 +263,14 @@ function startLiveBlock(tierName) {
     outputEl.appendChild(block);
     return {
         append(chunk) {
-            pre.textContent += chunk;
+            // Append a TextNode rather than `pre.textContent += chunk`
+            // — the latter re-reads + restringifies + re-sets all
+            // existing children on every chunk, which under a fast
+            // C-tier loop (one chunk per (newline) printf) was
+            // dropping line breaks and rendering "tick 0tick 10000…"
+            // horizontally instead of stacking vertically. TextNode
+            // append preserves every byte verbatim.
+            pre.appendChild(document.createTextNode(chunk));
             outputEl.scrollTop = outputEl.scrollHeight;
         },
         finalize(elapsedMs, kind, fullOutput) {
diff --git a/www/playground/app.js b/www/playground/app.js
index 2c61f8a..1f9d71e 100644
--- a/www/playground/app.js
+++ b/www/playground/app.js
@@ -263,7 +263,14 @@ function startLiveBlock(tierName) {
     outputEl.appendChild(block);
     return {
         append(chunk) {
-            pre.textContent += chunk;
+            // Append a TextNode rather than `pre.textContent += chunk`
+            // — the latter re-reads + restringifies + re-sets all
+            // existing children on every chunk, which under a fast
+            // C-tier loop (one chunk per (newline) printf) was
+            // dropping line breaks and rendering "tick 0tick 10000…"
+            // horizontally instead of stacking vertically. TextNode
+            // append preserves every byte verbatim.
+            pre.appendChild(document.createTextNode(chunk));
             outputEl.scrollTop = outputEl.scrollHeight;
         },
         finalize(elapsedMs, kind, fullOutput) {