diff --git a/src/off_topic.gleam b/src/off_topic.gleam
index f6d8ffc..5552ec7 100644
--- a/src/off_topic.gleam
+++ b/src/off_topic.gleam
@@ -105,6 +105,7 @@
 //// [window_hover](#window_hover "Dispatch the current window hover state"),
 //// [title](#title "Update the document title"),
 //// [meta](#meta "Upsert a document meta tag"),
+//// [body_class](#body_class "Set a class on the body tag"),
 //// [prevent_unload](#prevent_unload "Block the browser unload dialog")
 ////
 //// #### DOM
@@ -267,6 +268,49 @@ pub fn script() -> Element(msg) {
   )
 }
 
+// Run the application init function and start subscriptions.
+///
+/// The low-level counterpart to [application](#application), for use when you
+/// need to compose off_topic into an existing `lustre.application` manually.
+///
+/// See also: [update](#update), [view](#view), [application](#application)
+pub fn init(
+  flags flags: flags,
+  init init: fn(flags) -> #(model, Effect(message)),
+) -> #(Model(model, message), Effect(Message(message))) {
+  runtime.init(flags, init)
+}
+
+/// Handle a message in the subscription runtime.
+///
+/// The low-level counterpart to [application](#application), for use when you
+/// need to compose off_topic into an existing `lustre.application` manually.
+///
+/// See also: [init](#init), [view](#view), [application](#application)
+pub fn update(
+  model: Model(model, message),
+  message: Message(message),
+  subscriptions: fn(model) -> Subscription(message),
+  update: fn(model, message) -> #(model, Effect(message)),
+) -> #(Model(model, message), Effect(Message(message))) {
+  runtime.update(model, message, subscriptions, update)
+}
+
+/// Render the application view through the subscription runtime.
+///
+/// The low-level counterpart to [application](#application), for use when you
+/// need to compose off_topic into an existing `lustre.application` manually.
+/// On the server target, this also injects the `ot-client-runtime` web
+/// component into the rendered output.
+///
+/// See also: [init](#init), [update](#update), [application](#application)
+pub fn view(
+  model: Model(model, message),
+  view: fn(model) -> Element(message),
+) -> Element(Message(message)) {
+  runtime.view(model, view)
+}
+
 // -- MAKING SUBSCRIPTIONS ----------------------------------------------------
 
 /// A subscription that never fires.
@@ -938,6 +982,23 @@ pub fn meta(
   }
 }
 
+/// Add a CSS class to the document body.
+///
+/// The class is removed when the subscription stops or the class name changes.
+///
+/// See also: [title](#title)
+pub fn body_class(class_name: String) -> Subscription(message) {
+  case lustre.is_browser() {
+    True -> {
+      use _ <- from(watching: [dep(class_name)])
+      subscriptions.body_class(class_name)
+    }
+    False -> {
+      remote("off-topic/body-class", [json.string(class_name)], no_decoder())
+    }
+  }
+}
+
 /// Block the user from immediately leaving the page when enabled, showing a
 /// simple popup instead.
 ///
diff --git a/src/off_topic/internal/component.mjs b/src/off_topic/internal/component.mjs
index 6606a0b..1ec3efe 100644
--- a/src/off_topic/internal/component.mjs
+++ b/src/off_topic/internal/component.mjs
@@ -203,6 +203,7 @@ Object.assign(window.OffTopic.subscriptions, {
   "off-topic/session-storage": Subscriptions.session_storage,
   "off-topic/title": Subscriptions.title,
   "off-topic/meta": Subscriptions.meta,
+  "off-topic/body-class": Subscriptions.body_class,
   "off-topic/aria": Subscriptions.aria,
   "off-topic/pseudo-state": Subscriptions.pseudo_state,
   "off-topic/form-value": Subscriptions.form_value,
diff --git a/src/off_topic/internal/subscriptions.gleam b/src/off_topic/internal/subscriptions.gleam
index ad4a07a..240a410 100644
--- a/src/off_topic/internal/subscriptions.gleam
+++ b/src/off_topic/internal/subscriptions.gleam
@@ -99,6 +99,11 @@ pub fn meta(_name: String, _content: String) -> fn() -> Nil {
   fn() { Nil }
 }
 
+@external(javascript, "./subscriptions_ffi.mjs", "body_class")
+pub fn body_class(_name: String) -> fn() -> Nil {
+  fn() { Nil }
+}
+
 @external(javascript, "./subscriptions_ffi.mjs", "aria")
 pub fn aria(
   _name: String,
diff --git a/src/off_topic/internal/subscriptions_ffi.mjs b/src/off_topic/internal/subscriptions_ffi.mjs
index ecbff5d..5d6741f 100644
--- a/src/off_topic/internal/subscriptions_ffi.mjs
+++ b/src/off_topic/internal/subscriptions_ffi.mjs
@@ -271,6 +271,13 @@ export function meta(name, content) {
   return noop;
 }
 
+export function body_class(className) {
+  const classes = className.split(" ");
+
+  document.body.classList.add(...classes);
+  return () => document.body.classList.remove(...classes);
+}
+
 export function aria(name, value, _handler, root) {
   const host = root instanceof ShadowRoot ? root.host : root;
   const internals = host?.internals;