diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index 533958b5..1ad11c06 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -1212,7 +1212,7 @@
     "message": "Expose iframes via HTML[stylus-iframe]"
   },
   "optionsAdvancedExposeIframesNote": {
-    "message": "Enables writing iframe-specific CSS like 'html[stylus-iframe] h1 { display:none }'"
+    "message": "Exposes the top site domain in each iframe.\nEnables writing iframe-specific CSS like this:\nhtml[stylus-iframe$=\"twitter.com\"] h1 { display:none }"
   },
   "optionsAdvancedContextDelete": {
     "message": "Add 'Delete' in editor context menu"
diff --git a/background/background.js b/background/background.js
index eb3c59cb..fb245286 100644
--- a/background/background.js
+++ b/background/background.js
@@ -247,14 +247,38 @@ window.addEventListener('storageReady', function _() {
     }));
 });
 
+// *************************************************************************
+{
+  const getStylesForFrame = (msg, sender) => {
+    const stylesTask = getStyles(msg);
+    if (!sender || !sender.frameId) return stylesTask;
+    return Promise.all([
+      stylesTask,
+      getTab(sender.tab.id),
+    ]).then(([styles, tab]) => {
+      if (tab) styles.exposeIframes = tab.url.replace(/(\/\/[^/]*).*/, '$1');
+      return styles;
+    });
+  };
+  const updateAPI = (_, enabled) => {
+    window.API_METHODS.getStylesForFrame = enabled ? getStylesForFrame : getStyles;
+  };
+  prefs.subscribe(['exposeIframes'], updateAPI);
+  updateAPI(null, prefs.readOnlyValues.exposeIframes);
+}
+
 // *************************************************************************
 
 function webNavigationListener(method, {url, tabId, frameId}) {
-  getStyles({matchUrl: url, asHash: true}).then(styles => {
+  Promise.all([
+    getStyles({matchUrl: url, asHash: true}),
+    frameId && prefs.readOnlyValues.exposeIframes && getTab(tabId),
+  ]).then(([styles, tab]) => {
     if (method && URLS.supported(url) && tabId >= 0) {
       if (method === 'styleApply') {
         handleCssTransitionBug({tabId, frameId, url, styles});
       }
+      if (tab) styles.exposeIframes = tab.url.replace(/(\/\/[^/]*).*/, '$1');
       sendMessage({
         tabId,
         frameId,
diff --git a/content/apply.js b/content/apply.js
index 10f7554d..9038af87 100644
--- a/content/apply.js
+++ b/content/apply.js
@@ -43,7 +43,7 @@
       } catch (e) {}
     }
     const request = Object.assign({
-      method: 'getStyles',
+      method: 'getStylesForFrame',
       asHash: true,
       matchUrl,
     }, options);
@@ -143,9 +143,9 @@
     }
     exposeIframes = state;
     const attr = document.documentElement.getAttribute('stylus-iframe');
-    if (state && attr !== '') {
-      document.documentElement.setAttribute('stylus-iframe', '');
-    } else if (!state && attr === '') {
+    if (state && state !== attr) {
+      document.documentElement.setAttribute('stylus-iframe', state);
+    } else if (!state && attr !== undefined) {
       document.documentElement.removeAttribute('stylus-iframe');
     }
   }