diff --git a/fiat-html/disable-wasm-option.js b/fiat-html/disable-wasm-option.js
new file mode 100644
index 00000000000..2c907f8a611
--- /dev/null
+++ b/fiat-html/disable-wasm-option.js
@@ -0,0 +1,21 @@
+document.addEventListener('DOMContentLoaded', async function() {
+    const wasmCheckbox = document.getElementById('wasm');
+    const extraWasmLabel = document.getElementById('extraWasmLabel');
+    wasmCheckbox.disabled = true;  // Initially disable the checkbox
+
+    try {
+        if (await wasmFeatureDetect.tailCall()) {
+            wasmCheckbox.disabled = false;  // Re-enable the checkbox if tail calls are supported
+        } else {
+            wasmCheckbox.checked = false;
+
+            if (navigator.userAgent.includes('Firefox')) {
+                extraWasmLabel.innerHTML = '(enable <code>javascript.options.wasm_tail_calls</code> in <code>about:config</code>)';
+            } else {
+                extraWasmLabel.innerHTML = '(wasm tail calls not supported)';
+            }
+        }
+    } catch (error) {
+        console.error('Error checking wasm tail call support:', error);
+    }
+});
diff --git a/fiat-html/fiat-crypto.html b/fiat-html/fiat-crypto.html
index 69b37b48421..425c68af417 100644
--- a/fiat-html/fiat-crypto.html
+++ b/fiat-html/fiat-crypto.html
@@ -5,7 +5,7 @@
     <title>Fiat Cryptography Web Interface</title>
     <style>
         .error { color: red; white-space: pre-wrap; }
-        code { display: block; white-space: pre-wrap; border: 1px solid #ddd; padding: 10px; position: relative; }
+        /*code { display: block; white-space: pre-wrap; border: 1px solid #ddd; padding: 10px; position: relative; }*/
         .code-container {
             position: relative;
         }
@@ -15,7 +15,8 @@
             white-space: pre-wrap;
             border: 1px solid #ddd;
             padding: 10px;
-            margin-bottom: 20px; /* Space for the button, adjust as needed */
+            position: relative;
+            /*margin-bottom: 20px;*/ /* Space for the button, adjust as needed */
         }
 
         .copy-button {
@@ -50,8 +51,10 @@
                 <input type="radio" name="inputType" value="string" checked> Input String
             </label>
             <label>
-                <input type="checkbox" id="wasm" name="codeKind" value="wasm"> Use WASM
+                <input type="checkbox" id="wasm" name="codeKind" value="wasm"> Use WASM <span id="extraWasmLabel"></span>
             </label>
+        </div>
+        <div class="form-row">
             <button type="button" id="synthesizeButton">Synthesize</button>
             <button type="button" id="cancelButton" disabled>Cancel</button>
             <span id="status" class="status-span hidden"></span>
@@ -62,16 +65,19 @@
     <div id="error" class="error hidden"></div>
     <div id="output" class="hidden">
       <div id="stdoutContainer" class="code-container">
-        <code id="stdout"></code>
+        <code id="stdout" class="code"></code>
         <button class="copy-button" data-target="stdout">Copy</button>
       </div>
       <div id="stderrContainer" class="code-container hidden">
-        <code id="stderr"></code>
+        <code id="stderr" class="code"></code>
         <button class="copy-button" data-target="stderr">Copy</button>
       </div>
     </div>
     <script src="version.js"></script>
+    <script src="https://unpkg.com/wasm-feature-detect/dist/umd/index.js"></script>
     <script src="main.js"></script>
+    <!-- N.B. disable-wasm-option.js must come after main.js so that the wasm box is unchecked correctly after parsing argv -->
+    <script src="disable-wasm-option.js"></script>
     <script src="copy-button.js"></script>
 </body>
 </html>