diff --git a/apps/wordcraft-web/Cargo.toml b/apps/wordcraft-web/Cargo.toml
index 9152db333..4a2a9d014 100644
--- a/apps/wordcraft-web/Cargo.toml
+++ b/apps/wordcraft-web/Cargo.toml
@@ -29,4 +29,4 @@ rfd = { workspace = true }
 wasm-bindgen = "0.2"
 wasm-bindgen-futures = "0.4"
 js-sys = "0.3"
-web-sys = { version = "0.3", features = ["Window", "Document", "Element", "HtmlElement", "HtmlCanvasElement", "HtmlAnchorElement", "HtmlIFrameElement", "Blob", "BlobPropertyBag", "Url", "Location", "CssStyleDeclaration", "Event", "EventTarget", "BeforeUnloadEvent", "ClipboardEvent", "DataTransfer", "FileList", "File", "DomStringList", "IdbFactory", "IdbOpenDbRequest", "IdbRequest", "IdbDatabase", "IdbObjectStore", "IdbTransaction", "IdbTransactionMode", "MediaQueryList", "Storage"] }
+web-sys = { version = "0.3", features = ["Window", "Document", "Element", "HtmlElement", "HtmlCanvasElement", "HtmlAnchorElement", "HtmlIFrameElement", "Blob", "BlobPropertyBag", "Url", "Location", "CssStyleDeclaration", "Event", "EventTarget", "BeforeUnloadEvent", "ClipboardEvent", "DataTransfer", "FileList", "File", "DomStringList", "IdbFactory", "IdbOpenDbRequest", "IdbRequest", "IdbDatabase", "IdbObjectStore", "IdbTransaction", "IdbTransactionMode", "MediaQueryList", "Storage", "MessageEvent", "UrlSearchParams"] }
diff --git a/apps/wordcraft-web/src/host.rs b/apps/wordcraft-web/src/host.rs
new file mode 100644
index 000000000..6a0bd8859
--- /dev/null
+++ b/apps/wordcraft-web/src/host.rs
@@ -0,0 +1,158 @@
+//! The `postMessage` bridge to a same-origin parent page (`?host=parent`), for pages that embed
+//! WordCraft in an `<iframe>` and keep its files (a Nextcloud app, a personal file server). The
+//! same messages as GridCraft's and PdfCraft's bridges; each is a plain object with a `type`:
+//!
+//! - `wordcraft:ready` (to the parent): `{ version }`, the app is listening; send a file now.
+//! - `wordcraft:open` (from the parent): `{ name, bytes }`, bytes an `ArrayBuffer` or
+//!   `Uint8Array`. It opens like a document picked with File › Open or dropped on the window.
+//! - `wordcraft:save` (to the parent): `{ name, saveAs, bytes }` (a transferred `ArrayBuffer`) for
+//!   every file WordCraft would otherwise download: Save, Save As, File › Export and Export as
+//!   PDF. Save writes the opened file under its own name and format, as the desktop app does;
+//!   `saveAs` is true for Save As and File › Export. The parent stores it and reports failures
+//!   itself.
+//! - `wordcraft:dirty` (to the parent): `{ dirty }` whenever unsaved changes appear or go away.
+//! - `wordcraft:failed` (to the parent): `{ error }` when the app couldn't start.
+//!
+//! Only the parent window on the page's own origin is heard or answered. In host mode the parent
+//! guards leaving the page, and `?author=` names the user for comments and tracked changes.
+
+use std::cell::Cell;
+use std::rc::Rc;
+
+use wasm_bindgen::JsCast as _;
+use wasm_bindgen::JsValue;
+use wasm_bindgen::closure::Closure;
+use wordcraft_ui_egui::{Inbox, Services, WordApp};
+
+/// The longest user name taken from `?author=`.
+const MAX_AUTHOR_CHARS: usize = 200;
+
+#[derive(Clone)]
+pub struct Bridge {
+    window: web_sys::Window,
+    parent: web_sys::Window,
+    origin: String,
+}
+
+impl Bridge {
+    /// `None` when the page isn't framed (there is no parent to talk to), or when the parent is
+    /// on another origin: messages only go to this origin, so the browser would drop every save
+    /// without an error while the document showed as saved.
+    pub fn new() -> Option<Self> {
+        let window = web_sys::window()?;
+        let parent = window.parent().ok().flatten()?;
+        if js_sys::Object::is(&parent, &window) {
+            return None;
+        }
+        let origin = window.location().origin().ok()?;
+        // Reading another origin's location throws, which is `None` too.
+        if parent.location().origin().ok()? != origin {
+            return None;
+        }
+        Some(Self { window, parent, origin })
+    }
+
+    /// Hand saved and exported files to the parent instead of downloading them, report unsaved
+    /// changes to it, and hear the files it sends.
+    pub fn connect(&self, services: &mut Services, inbox: Inbox, ctx: egui::Context) {
+        self.listen(inbox, ctx);
+        let bridge = self.clone();
+        services.host_save = Some(Box::new(move |name: &str, bytes: &[u8], save_as: bool| bridge.post_save(name, bytes, save_as)));
+        // Anything else that would download goes to the parent too.
+        let bridge = self.clone();
+        services.download = Some(Box::new(move |name: &str, bytes: &[u8]| bridge.post_save(name, bytes, false)));
+        // The browser's save picker is never used, so no save finishes later.
+        services.async_saves = None;
+        let bridge = self.clone();
+        let reported: Rc<Cell<Option<bool>>> = Rc::default();
+        services.on_dirty = Some(Box::new(move |dirty: bool| {
+            if reported.get() != Some(dirty) && bridge.send(&[("type", "wordcraft:dirty".into()), ("dirty", dirty.into())]) {
+                reported.set(Some(dirty));
+            }
+        }));
+    }
+
+    /// `?author=`: the host's name for the user signs comments and tracked changes.
+    pub fn set_author(app: &mut WordApp, author: &str) {
+        let author: String = author.trim().chars().take(MAX_AUTHOR_CHARS).collect();
+        if !author.is_empty() {
+            app.session.author = author;
+        }
+    }
+
+    pub fn post_ready(&self) {
+        self.send(&[("type", "wordcraft:ready".into()), ("version", env!("CARGO_PKG_VERSION").into())]);
+    }
+
+    /// The app couldn't start (no WebGPU or WebGL2, for example): let the parent say so.
+    pub fn post_failed(&self, error: &str) {
+        self.send(&[("type", "wordcraft:failed".into()), ("error", error.into())]);
+    }
+
+    fn post_save(&self, path: &str, bytes: &[u8], save_as: bool) -> Result<(), String> {
+        let name = std::path::Path::new(path).file_name().map(|n| n.to_string_lossy().to_string()).unwrap_or_else(|| path.to_string());
+        let buffer = js_sys::Uint8Array::from(bytes).buffer();
+        let message = message(&[
+            ("type", "wordcraft:save".into()),
+            ("name", name.as_str().into()),
+            ("saveAs", save_as.into()),
+            ("bytes", buffer.clone().into()),
+        ])
+        .ok_or_else(|| "couldn't build the message".to_string())?;
+        self.parent
+            .post_message_with_transfer(&message, &self.origin, &js_sys::Array::of1(&buffer))
+            .map_err(|e| e.as_string().unwrap_or_else(|| format!("{e:?}")))
+    }
+
+    /// Hear `wordcraft:open` from the parent and queue the file like a dropped one.
+    fn listen(&self, inbox: Inbox, ctx: egui::Context) {
+        let parent = self.parent.clone();
+        let origin = self.origin.clone();
+        let on_message = Closure::<dyn FnMut(web_sys::MessageEvent)>::new(move |event: web_sys::MessageEvent| {
+            if event.origin() != origin || !event.source().is_some_and(|s| js_sys::Object::is(&s, &parent)) {
+                return;
+            }
+            let data = event.data();
+            if string(&data, "type").as_deref() != Some("wordcraft:open") {
+                return;
+            }
+            let name = string(&data, "name").filter(|n| !n.trim().is_empty()).unwrap_or_else(|| "Document.docx".to_string());
+            let Some(bytes) = bytes(&data) else { return };
+            inbox.lock().unwrap_or_else(|e| e.into_inner()).push((name, bytes));
+            ctx.request_repaint();
+        });
+        if self.window.add_event_listener_with_callback("message", on_message.as_ref().unchecked_ref()).is_ok() {
+            // The listener lives as long as the page.
+            on_message.forget();
+        }
+    }
+
+    /// Post a message to the parent; false if it couldn't be sent.
+    fn send(&self, fields: &[(&str, JsValue)]) -> bool {
+        message(fields).is_some_and(|m| self.parent.post_message(&m, &self.origin).is_ok())
+    }
+}
+
+fn message(fields: &[(&str, JsValue)]) -> Option<js_sys::Object> {
+    let m = js_sys::Object::new();
+    for (key, value) in fields {
+        js_sys::Reflect::set(&m, &JsValue::from_str(key), value).ok()?;
+    }
+    Some(m)
+}
+
+fn string(data: &JsValue, key: &str) -> Option<String> {
+    js_sys::Reflect::get(data, &JsValue::from_str(key)).ok()?.as_string()
+}
+
+/// `bytes` as an `ArrayBuffer` or `Uint8Array`.
+fn bytes(data: &JsValue) -> Option<Vec<u8>> {
+    let bytes = js_sys::Reflect::get(data, &JsValue::from_str("bytes")).ok()?;
+    if let Some(array) = bytes.dyn_ref::<js_sys::Uint8Array>() {
+        Some(array.to_vec())
+    } else if bytes.is_instance_of::<js_sys::ArrayBuffer>() {
+        Some(js_sys::Uint8Array::new(&bytes).to_vec())
+    } else {
+        None
+    }
+}
diff --git a/apps/wordcraft-web/src/main.rs b/apps/wordcraft-web/src/main.rs
index 39c2f981f..d9592222d 100644
--- a/apps/wordcraft-web/src/main.rs
+++ b/apps/wordcraft-web/src/main.rs
@@ -8,6 +8,9 @@
 //! browser file picker (bytes arrive through `Services::inbox`); Save and Export download.
 //! Preferences (theme, interface size, editing options…) are kept in `localStorage` (`prefs`).
 //! URL flags: `?webgl` forces WebGL2; `?sample` opens the sample document.
+//! `?host=parent` (inside an `<iframe>`) talks to a same-origin parent page instead: it sends the
+//! document to open and stores what WordCraft saves (`host`), and `&author=` names the user for
+//! comments and tracked changes.
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 
 #[cfg(any(target_arch = "wasm32", test))]
@@ -15,6 +18,8 @@ mod prefs;
 #[cfg(any(target_arch = "wasm32", test))]
 mod save_fallback;
 
+#[cfg(target_arch = "wasm32")]
+mod host;
 #[cfg(target_arch = "wasm32")]
 mod web;
 
diff --git a/apps/wordcraft-web/src/web.rs b/apps/wordcraft-web/src/web.rs
index b2e698cab..633dcf71b 100644
--- a/apps/wordcraft-web/src/web.rs
+++ b/apps/wordcraft-web/src/web.rs
@@ -27,9 +27,15 @@ pub fn start() {
             log::error!("missing <canvas id=\"{CANVAS_ID}\">");
             return;
         };
+        // `?host=parent`: a same-origin page framing WordCraft opens documents in it and stores
+        // what it saves (`host`). That page guards leaving it, so this one doesn't.
+        let bridge = if query_param("host").as_deref() == Some("parent") { crate::host::Bridge::new() } else { None };
+        let app_bridge = bridge.clone();
         let dirty = Rc::new(Cell::new(false));
         let saves = AsyncSaves::default();
-        if let Err(e) = guard_unload(dirty.clone(), saves.clone()) {
+        if bridge.is_none()
+            && let Err(e) = guard_unload(dirty.clone(), saves.clone())
+        {
             log::error!("no unsaved-changes guard: {e}");
         }
         let mut options = eframe::WebOptions::default();
@@ -56,12 +62,17 @@ pub fn start() {
                     // The browser's locale picks the new document's paper, the units and the
                     // decimal separator (#838).
                     let region = wordcraft_ui_egui::i18n::system_region();
-                    let sample = query().contains("sample");
+                    // In host mode the parent sends the document to edit; no sample meanwhile.
+                    let sample = app_bridge.is_none() && query().contains("sample");
                     let doc = if sample { wordcraft_engine::sample::sample_document() } else { wordcraft_engine::blank_document(&region) };
                     let mut session = Session::new(doc);
                     session.system_region = region;
                     let messages = Rc::new(RefCell::new(Vec::new()));
-                    let mut app = WordApp::new(session, services(inbox.clone(), cc.egui_ctx.clone(), dirty, saves.clone(), messages.clone()));
+                    let mut services = services(inbox.clone(), cc.egui_ctx.clone(), dirty, saves.clone(), messages.clone());
+                    if let Some(bridge) = &app_bridge {
+                        bridge.connect(&mut services, inbox.clone(), cc.egui_ctx.clone());
+                    }
+                    let mut app = WordApp::new(session, services);
                     // Pasted pictures wait apart from dropped files: the app inserts one only when the page is the edit target (#1349).
                     app.services.pasted = Some(pasted);
                     app.autosave = false;
@@ -79,10 +90,19 @@ pub fn start() {
                     if !sample {
                         app.start_screen();
                     }
+                    if let Some(bridge) = &app_bridge {
+                        if let Some(author) = query_param("author") {
+                            crate::host::Bridge::set_author(&mut app, &author);
+                        }
+                        bridge.post_ready();
+                    }
                     Ok(Box::new(WebShell { app, inbox, messages, prefs }))
                 }),
             )
             .await;
+        if let (Err(e), Some(bridge)) = (&result, &bridge) {
+            bridge.post_failed(&format!("{e:?}"));
+        }
         if let Some(el) = document.get_element_by_id(LOADING_ID) {
             match result {
                 Ok(()) => el.remove(),
@@ -96,6 +116,10 @@ fn query() -> String {
     web_sys::window().and_then(|w| w.location().search().ok()).unwrap_or_default()
 }
 
+fn query_param(key: &str) -> Option<String> {
+    web_sys::UrlSearchParams::new_with_str(&query()).ok()?.get(key)
+}
+
 /// Wraps the app to read dropped files asynchronously (browsers can't read them synchronously)
 /// and feed them through the inbox.
 struct WebShell {
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 5dede3611..9a52d0ad8 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -127,6 +127,12 @@ pub struct Services {
     /// #1315). The hook starts a ticket for such a save and reports how it ended; until then the
     /// document's save isn't confirmed, and a New/Open held back by the Save Changes prompt waits.
     pub async_saves: Option<async_saves::AsyncSaves>,
+    /// Web, framed by a page that keeps the files (`?host=parent`): set alongside `download`,
+    /// Save, Save As and Export hand the bytes to that page instead, with whether it was Save As
+    /// ([`HostSaveFn`]). Save then writes the opened file under its own name and format, as on
+    /// the desktop, and a Save or Save As that keeps everything makes that name the document's
+    /// file, so the next Save goes to it.
+    pub host_save: Option<HostSaveFn>,
     /// Web: told whether the document has unsaved changes after each pass, for the browser's
     /// leave-page guard (`beforeunload` runs between frames and can't ask the app).
     pub on_dirty: Option<Box<dyn Fn(bool)>>,
@@ -223,6 +229,10 @@ pub enum DefaultApp {
 /// Files delivered asynchronously.
 pub type Inbox = std::sync::Arc<std::sync::Mutex<Vec<(String, Vec<u8>)>>>;
 
+/// [`Services::host_save`]: `(file name, bytes, save_as)`. An error means the page didn't get
+/// the file, and the save failed.
+pub type HostSaveFn = Box<dyn Fn(&str, &[u8], bool) -> Result<(), String>>;
+
 /// Whether a pasted picture belongs in the document: the page has keyboard focus, and no
 /// Backstage or dialog is open (#1349). A focused text field (comment, Find box) means the page
 /// does not have focus, so `canvas_focused` already covers it; egui's own "wants keyboard input"
@@ -1004,7 +1014,7 @@ impl WordApp {
         }
         // Web: saving and exporting become downloads.
         if self.services.download.is_some() && matches!(id, "file.save" | "file.saveAs" | "file.exportPdf" | "file.exportPng") {
-            let name = params.get("path").and_then(Value::as_str).map(str::to_string).unwrap_or_else(|| self.default_save_name());
+            let name = params.get("path").and_then(Value::as_str).map(str::to_string).unwrap_or_else(|| self.web_save_name(id));
             let name = if id == "file.exportPdf" && !name.ends_with(".pdf") { format!("{name}.pdf") } else { name };
             // A save with a password encrypts this and later saves (`file.save`'s `password`).
             if matches!(id, "file.save" | "file.saveAs")
@@ -1029,9 +1039,12 @@ impl WordApp {
             if let Some(saves) = &saves {
                 let _ = saves.take_started();
             }
-            if let Some(d) = &self.services.download
-                && let Err(e) = d(&name, &bytes)
-            {
+            let delivered = match (&self.services.host_save, &self.services.download) {
+                (Some(host), _) => host(&name, &bytes, id == "file.saveAs"),
+                (None, Some(download)) => download(&name, &bytes),
+                (None, None) => Ok(()),
+            };
+            if let Err(e) = delivered {
                 let msg = i18n::fmt(tl!("Couldn't download {name}: {error}"), &[("name", &name), ("error", &e)]);
                 log::error!("download of {name} failed: {e}");
                 self.status(msg.clone());
@@ -1042,6 +1055,10 @@ impl WordApp {
             let marked_saved = matches!(id, "file.save" | "file.saveAs") && keeps_everything(&name);
             if marked_saved {
                 self.session.dirty = false;
+                // The host page keeps the file under this name now, as a desktop save would.
+                if self.services.host_save.is_some() {
+                    self.session.path = Some(std::path::PathBuf::from(&name));
+                }
             }
             // The browser's picker writes the file later (#1315): until it says how that went,
             // the document's save isn't confirmed.
@@ -1931,6 +1948,20 @@ impl WordApp {
 
     /// The name Save suggests: the open file's own Word format (so a .docm keeps its macros),
     /// otherwise .docx.
+    /// The name a browser Save or Export without a name uses. A host page that keeps the files
+    /// ([`Services::host_save`]) gets Save back under the opened file's own name and format, as
+    /// the desktop app writes it, except an import-only Word 97–2003 file (#2445).
+    fn web_save_name(&self, id: &str) -> String {
+        if id == "file.save"
+            && self.services.host_save.is_some()
+            && !self.legacy_word_path()
+            && let Some(name) = self.session.path.as_ref().and_then(|p| p.file_name())
+        {
+            return name.to_string_lossy().to_string();
+        }
+        self.default_save_name()
+    }
+
     fn default_save_name(&self) -> String {
         let ext = self.session.path.as_ref().and_then(|p| p.extension()).map(|e| e.to_string_lossy().to_string());
         match ext {
@@ -6761,6 +6792,80 @@ mod tests {
         }
     }
 
+    /// Web inside a page that keeps the files: the app with `host_save` recording each file it
+    /// is handed (name, Save As, size).
+    fn host_app(fail: bool) -> (WordApp, std::rc::Rc<std::cell::RefCell<Vec<(String, bool, usize)>>>) {
+        let mut a = typed();
+        let got = std::rc::Rc::new(std::cell::RefCell::new(Vec::new()));
+        let seen = got.clone();
+        a.services.download = Some(Box::new(|_, _| Err("downloads go to the host page".into())));
+        a.services.host_save = Some(Box::new(move |name: &str, bytes: &[u8], save_as: bool| {
+            if fail {
+                return Err("the page is gone".into());
+            }
+            seen.borrow_mut().push((name.to_string(), save_as, bytes.len()));
+            Ok(())
+        }));
+        (a, got)
+    }
+
+    /// Open `name` as the host page sends it (bytes of the typed document in that format).
+    fn host_open(a: &mut WordApp, name: &str) {
+        let bytes = wordcraft_engine::io::save_bytes_with(name, &typed().session.doc, None, &Default::default()).unwrap();
+        a.execute("file.open", json!({"path": name, "data": wordcraft_engine::cmd::insert::base64_encode(&bytes)})).unwrap();
+        a.run("text.insert", json!({"text": "Edited "})).unwrap();
+        assert!(a.session.dirty);
+    }
+
+    /// Save hands the opened file back under its own name and format; Save As says so and
+    /// becomes the file the next Save writes.
+    #[test]
+    fn host_saves_go_back_to_the_opened_file() {
+        let (mut a, got) = host_app(false);
+        host_open(&mut a, "report.odt");
+        a.execute("file.save", json!({})).unwrap();
+        assert!(!a.session.dirty);
+        a.run("text.insert", json!({"text": "More "})).unwrap();
+        a.execute("file.saveAs", json!({"path": "report copy.docx"})).unwrap();
+        assert!(!a.session.dirty);
+        a.run("text.insert", json!({"text": "Again "})).unwrap();
+        a.execute("file.save", json!({})).unwrap();
+        let got = got.borrow();
+        let calls: Vec<(&str, bool)> = got.iter().map(|(n, s, _)| (n.as_str(), *s)).collect();
+        assert_eq!(calls, [("report.odt", false), ("report copy.docx", true), ("report copy.docx", false)]);
+        assert!(got.iter().all(|(_, _, len)| *len > 0));
+    }
+
+    /// A Markdown file goes back as Markdown, which doesn't keep everything: the document stays
+    /// unsaved, as on the desktop.
+    #[test]
+    fn host_save_of_markdown_keeps_its_format_and_stays_unsaved() {
+        let (mut a, got) = host_app(false);
+        host_open(&mut a, "notes.md");
+        a.execute("file.save", json!({})).unwrap();
+        assert_eq!(got.borrow().first().map(|(n, s, _)| (n.as_str(), *s)), Some(("notes.md", false)));
+        assert!(a.session.dirty);
+        assert_eq!(a.session.path.as_deref(), Some(std::path::Path::new("notes.md")));
+    }
+
+    /// An import-only Word 97–2003 file is saved as a .docx, never back over itself (#2445).
+    #[test]
+    fn host_save_of_a_legacy_word_file_writes_a_docx() {
+        let (mut a, got) = host_app(false);
+        a.session.path = Some("old.doc".into());
+        a.execute("file.save", json!({})).unwrap();
+        assert_eq!(got.borrow().first().map(|(n, _, _)| n.as_str()), Some("old.docx"));
+    }
+
+    /// The page not getting the file is a failed save: the document keeps its changes.
+    #[test]
+    fn a_host_save_that_fails_leaves_the_document_unsaved() {
+        let (mut a, _) = host_app(true);
+        host_open(&mut a, "report.docx");
+        assert!(a.execute("file.save", json!({})).is_err());
+        assert!(a.session.dirty);
+    }
+
     /// Macro-enabled documents and templates keep everything (#172), so once saved in this
     /// session AutoSave covers them like a .docx.
     #[test]