diff --git a/internal/web/posts_test.go b/internal/web/posts_test.go
index 8e798a1..1b63317 100644
--- a/internal/web/posts_test.go
+++ b/internal/web/posts_test.go
@@ -458,6 +458,8 @@ func TestQuestionPageRendersNestedPostControls(t *testing.T) {
 		"Shop response",
 		"Edited",
 		`action="/posts"`,
+		`data-submit-once`,
+		`data-submit-button`,
 		`action="/posts/` + root.ID + `/edit"`,
 		`action="/posts/` + homeownerReply.ID + `/edit"`,
 		`>The model number is 123A.</textarea>`,
diff --git a/internal/web/server_test.go b/internal/web/server_test.go
index 988ae49..5a9393d 100644
--- a/internal/web/server_test.go
+++ b/internal/web/server_test.go
@@ -168,6 +168,16 @@ func TestRegisterLoginAsk(t *testing.T) {
 	if rec.Code != 200 {
 		t.Fatalf("submit form %d", rec.Code)
 	}
+	for _, want := range []string{
+		`src="/static/app.js"`,
+		`id="submit-progress"`,
+		`data-submit-once`,
+		`data-submit-button`,
+	} {
+		if !strings.Contains(rec.Body.String(), want) {
+			t.Fatalf("submit form missing %q: %s", want, rec.Body.String())
+		}
+	}
 	csrf := csrfFrom(rec.Body.String())
 	form := strings.NewReader("_csrf=" + csrf + "&title=Leaky+faucet&body=Drip+all+night.&city=Oakland")
 	req = httptest.NewRequest(http.MethodPost, "/submit", form)
diff --git a/static/app.css b/static/app.css
index 38898e0..c5fa9f7 100644
--- a/static/app.css
+++ b/static/app.css
@@ -41,6 +41,31 @@ body {
   background-size: 100% 100%, 48px 48px;
 }
 
+.submit-progress {
+  position: fixed;
+  inset: 0 0 auto;
+  z-index: 100;
+  height: 3px;
+  overflow: hidden;
+  pointer-events: none;
+}
+
+.submit-progress[hidden] { display: none; }
+
+.submit-progress-bar {
+  display: block;
+  width: 35%;
+  height: 100%;
+  background: var(--signal);
+  box-shadow: 0 0 12px var(--signal);
+  animation: submit-progress 900ms ease-in-out infinite;
+}
+
+@keyframes submit-progress {
+  from { transform: translateX(-100%); }
+  to { transform: translateX(300%); }
+}
+
 img, svg { display: block; }
 
 a {
@@ -253,6 +278,12 @@ a {
 
 .btn-primary:hover { filter: brightness(1.08); }
 
+.btn:disabled {
+  cursor: wait;
+  opacity: 0.65;
+  filter: none;
+}
+
 .btn-ghost {
   background: transparent;
   color: var(--ink);
@@ -763,6 +794,10 @@ input:focus, textarea:focus, .btn:focus-visible, .chip:focus-visible, .vote-btn:
 
 @media (prefers-reduced-motion: reduce) {
   .btn-primary:hover { filter: none; }
+  .submit-progress-bar {
+    width: 100%;
+    animation: none;
+  }
 }
 
 .admin-users { margin-top: 20px; overflow-x: auto; }
diff --git a/static/app.js b/static/app.js
new file mode 100644
index 0000000..1ec32a4
--- /dev/null
+++ b/static/app.js
@@ -0,0 +1,58 @@
+(() => {
+  const formSelector = "form[data-submit-once]";
+
+  function progressIndicator() {
+    return document.getElementById("submit-progress");
+  }
+
+  function resetForm(form) {
+    form.removeAttribute("aria-busy");
+    delete form.dataset.submitting;
+
+    const button = form.querySelector("[data-submit-button]");
+    if (!button) {
+      return;
+    }
+    button.disabled = false;
+    button.removeAttribute("aria-disabled");
+    if (button.dataset.idleLabel) {
+      button.textContent = button.dataset.idleLabel;
+      delete button.dataset.idleLabel;
+    }
+  }
+
+  document.addEventListener("submit", (event) => {
+    const form = event.target.closest(formSelector);
+    if (!form) {
+      return;
+    }
+    if (form.dataset.submitting === "true") {
+      event.preventDefault();
+      return;
+    }
+
+    form.dataset.submitting = "true";
+    form.setAttribute("aria-busy", "true");
+
+    const button = event.submitter || form.querySelector("[data-submit-button]");
+    if (button) {
+      button.dataset.idleLabel = button.textContent;
+      button.textContent = form.dataset.submittingLabel || "Posting…";
+      button.disabled = true;
+      button.setAttribute("aria-disabled", "true");
+    }
+
+    const progress = progressIndicator();
+    if (progress) {
+      progress.hidden = false;
+    }
+  });
+
+  window.addEventListener("pageshow", () => {
+    document.querySelectorAll(formSelector).forEach(resetForm);
+    const progress = progressIndicator();
+    if (progress) {
+      progress.hidden = true;
+    }
+  });
+})();
diff --git a/templates/base.html b/templates/base.html
index f7a32d0..591038f 100644
--- a/templates/base.html
+++ b/templates/base.html
@@ -11,8 +11,13 @@
   <link href="https://fonts.googleapis.com/css2?family=Archivo:ital,wght@0,400;0,500;0,600;1,400&family=IBM+Plex+Mono:wght@400;500;600&display=swap" rel="stylesheet">
   <link rel="stylesheet" href="/static/app.css">
   <script src="/static/htmx.min.js" defer></script>
+  <script src="/static/app.js" defer></script>
 </head>
 <body>
+  <div id="submit-progress" class="submit-progress" role="progressbar"
+    aria-label="Posting" aria-valuetext="Posting" hidden>
+    <span class="submit-progress-bar"></span>
+  </div>
   <a class="skip" href="#main">Skip to content</a>
   <header class="top">
     <div class="top-inner">
diff --git a/templates/partials/_post.html b/templates/partials/_post.html
index d01569f..95e1c26 100644
--- a/templates/partials/_post.html
+++ b/templates/partials/_post.html
@@ -3,13 +3,14 @@
   {{if canReply .User .Root}}
     <details class="post-composer">
       <summary>Reply</summary>
-      <form class="post-form" method="post" action="/posts">
+      <form class="post-form" method="post" action="/posts"
+        data-submit-once data-submitting-label="Posting…">
         <input type="hidden" name="_csrf" value="{{.CSRF}}">
         <input type="hidden" name="parent_id" value="{{.Post.ID}}">
         <label for="reply-{{.Post.ID}}">Reply to {{.Post.AuthorName}}</label>
         <textarea id="reply-{{.Post.ID}}" name="body" rows="5" required maxlength="12000"></textarea>
         <div class="post-form-actions">
-          <button type="submit" class="btn btn-primary">Post reply</button>
+          <button type="submit" class="btn btn-primary" data-submit-button>Post reply</button>
           <button type="reset" class="btn btn-ghost"
             onclick="this.closest('details').removeAttribute('open')">Cancel</button>
         </div>
diff --git a/templates/submit.html b/templates/submit.html
index ce8668f..bfb2175 100644
--- a/templates/submit.html
+++ b/templates/submit.html
@@ -4,7 +4,8 @@
   <h1>Ask a question</h1>
   <p class="lede">It lands on today’s hunt (Pacific time). People vote; the ranking resets at midnight PT.</p>
   {{if .Error}}<p class="banner error" role="alert">{{.Error}}</p>{{end}}
-  <form class="ask" method="post" action="/submit">
+  <form class="ask" method="post" action="/submit"
+    data-submit-once data-submitting-label="Posting…">
     <input type="hidden" name="_csrf" value="{{.CSRF}}">
     <label for="title">Title</label>
     <input id="title" name="title" type="text" required maxlength="120" value="{{.TitleVal}}" placeholder="Water heater popping after showers">
@@ -12,7 +13,7 @@
     <textarea id="body" name="body" rows="8" required maxlength="8000" placeholder="Age of the house, what you already tried, where you are in the Bay if it helps.">{{.BodyVal}}</textarea>
     <label for="city">City <span class="optional">(optional)</span></label>
     <input id="city" name="city" type="text" maxlength="80" value="{{.CityVal}}" placeholder="Oakland">
-    <button type="submit" class="btn btn-primary">Submit to today’s hunt</button>
+    <button type="submit" class="btn btn-primary" data-submit-button>Submit to today’s hunt</button>
   </form>
 </main>
 {{template "footer" .}}