1 files changed, 38 insertions, 13 deletions
diff --git a/src/main.rs b/src/main.rs
index a5b7ce0..58431a2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -265,17 +265,18 @@ async fn service<C: Controller>(
         });
 
     // we rely on CSP to thwart XSS attacks, all modern browsers support it
-    resp.headers_mut().insert(
-        header::CONTENT_SECURITY_POLICY,
-        format!(
-            "default-src 'self'; frame-src {}; script-src {}; style-src {}",
-            frame_csp,
-            script_csp,
-            include_str!("static/style.css.sha"),
-        )
-        .parse()
-        .unwrap(),
-    );
+    resp.headers_mut()
+        .entry(header::CONTENT_SECURITY_POLICY)
+        .or_insert_with(|| {
+            format!(
+                "default-src 'self'; frame-src {}; script-src {}; style-src {}",
+                frame_csp,
+                script_csp,
+                include_str!("static/style.css.sha"),
+            )
+            .parse()
+            .unwrap()
+        });
     Ok(resp)
 }
 
@@ -526,16 +527,40 @@ impl Context {
     }
 }
 
-fn render_markdown(input: &str, page: &mut Page) {
+#[derive(PartialEq)]
+enum RenderMode {
+    View,
+    Preview,
+}
+
+fn render_markdown(input: &str, page: &mut Page, _mode: RenderMode) {
     let parser = Parser::new_ext(input, Options::all());
     page.body.push_str("<div class=markdown-output>");
     html::push_html(&mut page.body, parser);
     page.body.push_str("</div>");
 }
 
-fn get_renderer(path: &Path) -> Option<fn(&str, &mut Page)> {
+fn embed_html_as_iframe(input: &str, page: &mut Page, mode: RenderMode) {
+    if mode == RenderMode::View {
+        page.body.push_str("<iframe src='?action=run'></iframe>");
+        page.frame_src = Some("'self'");
+    } else {
+        page.body
+            .push_str("<div class=note>Note that JavaScript does not work in the preview.</div>");
+        // sandbox=allow-scripts wouldn't work because the strict parent page CSP still applies
+
+        // The sandbox attribute makes browsers treat the embedded page as a unique origin.
+        page.body.push_str(&format!(
+            "<iframe srcdoc='{}' sandbox></iframe>",
+            html_escape(input)
+        ));
+    }
+}
+
+fn get_renderer(path: &Path) -> Option<fn(&str, &mut Page, RenderMode)> {
     match path.extension().map(|e| e.to_str().unwrap()) {
         Some("md") => Some(render_markdown),
+        Some("html") => Some(embed_html_as_iframe),
         _ => None,
     }
 }