feat: Improve before_mount docs and do runtime debug check

See commit code for explanation.

src/app.rs

@@ -246,6 +246,9 @@ impl<Ms, Mdl, ElC: View<Ms> + 'static, GMs: 'static> App<Ms, Mdl, ElC, GMs> {
             // not entirely correct).
             // TODO: 1) Please refer to [issue #277](https://github.com/seed-rs/seed/issues/277)
             let mut dom_nodes: El<Ms> = (&self.cfg.mount_point).into();
+            if cfg!(debug_assertions) {
+                dom_nodes.warn_about_script_tags();
+            }
             dom_nodes.strip_ws_nodes_from_self_and_children();
 
             // Replace the root dom with a placeholder tag and move the children from the root element

src/app/builder/before_mount.rs

@@ -83,8 +83,14 @@ impl BeforeMount {
     /// mount_point("another_id")
     ///
     /// // argument is `HTMLElement`
-    /// // NOTE: Be careful with mounting into body,
-    /// // it can cause hard-to-debug bugs when there are other scripts in the body.
+    ///
+    /// // NOTE: Be careful with mounting into body!
+    /// // If you render directly into document.body, you risk collisions
+    /// // with scripts that do something with it (e.g. Google Font Loader or
+    /// // third party browser extensions) which produce very weird and hard
+    /// // to debug errors in production.
+    /// // (from https://github.com/facebook/create-react-app/issues/1568)
+    ///
     /// mount_point(seed::body())
     ///
     /// // argument is `Element`

src/virtual_dom/node.rs

@@ -168,6 +168,13 @@ impl<Ms> Node<Ms> {
             Node::Empty => (),
         }
     }
+
+    #[cfg(debug_assertions)]
+    pub fn warn_about_script_tags(&self) {
+        if let Node::Element(e) = self {
+            e.warn_about_script_tags();
+        }
+    }
 }
 
 impl<Ms: 'static, OtherMs: 'static> MessageMapper<Ms, OtherMs> for Node<Ms> {

src/virtual_dom/node/el.rs

@@ -211,6 +211,24 @@ impl<Ms> El<Ms> {
             .collect()
     }
 
+    #[cfg(debug_assertions)]
+    /// Warn user about potential bugs when having scripts and `Takeover` mount type.
+    pub fn warn_about_script_tags(&self) {
+        let script_found = match &self.tag {
+            Tag::Script => true,
+            Tag::Custom(tag) if tag == "script" => true,
+            _ => false,
+        };
+        if script_found {
+            error!("Script tag found inside mount point! \
+                    Please check https://docs.rs/seed/latest/seed/app/builder/struct.Builder.html#examples");
+        }
+
+        for child in &self.children {
+            child.warn_about_script_tags();
+        }
+    }
+
     /// Remove websys nodes.
     pub fn strip_ws_nodes_from_self_and_children(&mut self) {
         self.node_ws.take();