diff --git a/src/App.tsx b/src/App.tsx
index 7ce4a40..a0cfbf2 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -1,8 +1,10 @@
 import { useState, useEffect, useLayoutEffect, useCallback, useRef, useMemo } from "react";
 import { open } from "@tauri-apps/plugin-dialog";
+import { openUrl } from "@tauri-apps/plugin-opener";
 import { readTextFile } from "@tauri-apps/plugin-fs";
 import { getCurrentWindow } from "@tauri-apps/api/window";
 import { getCurrentWebview } from "@tauri-apps/api/webview";
+import { invoke } from "@tauri-apps/api/core";
 import { listen } from "@tauri-apps/api/event";
 import MarkdownIt from "markdown-it";
 import hljs from "highlight.js";
@@ -337,6 +339,21 @@ function App() {
     };
   }, [activeTab, uiZoom]);
 
+  // Open external links in default OS browser
+  useEffect(() => {
+    const handleLinkClick = (e: MouseEvent) => {
+      const anchor = (e.target as HTMLElement).closest('a[href]') as HTMLAnchorElement | null;
+      if (!anchor) return;
+      const href = anchor.getAttribute('href');
+      if (href && /^https?:\/\//.test(href)) {
+        e.preventDefault();
+        openUrl(href);
+      }
+    };
+    document.addEventListener('click', handleLinkClick);
+    return () => document.removeEventListener('click', handleLinkClick);
+  }, []);
+
   // Clear selection overlay when context menu closes
   useEffect(() => {
     if (!contextMenu) setSelectionRects(prev => prev.length > 0 ? [] : prev);
@@ -514,25 +531,32 @@ function App() {
   }, [parseHeadings]);
 
   // Handle file opened via OS file association (e.g. double-clicking a .md file)
+  useEffect(() => {
+    invoke<{ path: string; content: string } | null>('get_cli_file').then(result => {
+      if (!result) return;
+      const title = result.path.split(/[/\\]/).pop() || 'Untitled';
+      const newTab: Tab = { id: Date.now().toString(), title, content: result.content, path: result.path };
+      setTabs(prev => [...prev, newTab]);
+      setActiveTabId(newTab.id);
+      parseHeadings(result.content);
+    }).catch(err => console.error('Failed to open file from CLI args:', err));
+  }, []);
+
+  // Handle files opened from a second instance (single-instance plugin forwards them here)
   useEffect(() => {
     let unlisten: (() => void) | undefined;
-    listen<string>('open-file', async (event) => {
-      const filePath = event.payload;
-      try {
-        const existing = tabsRef.current.find(t => t.path === filePath);
-        if (existing) {
-          setActiveTabId(existing.id);
-          parseHeadings(existing.content);
-        } else {
-          const content = await readTextFile(filePath);
-          const title = filePath.split(/[/\\]/).pop() || 'Untitled';
-          const newTab: Tab = { id: Date.now().toString(), title, content, path: filePath };
-          setTabs(prev => [...prev, newTab]);
-          setActiveTabId(newTab.id);
-          parseHeadings(content);
-        }
-      } catch (err) {
-        console.error('Failed to open file from OS:', err);
+    listen<{ path: string; content: string }>('open-file', (event) => {
+      const { path: filePath, content } = event.payload;
+      const existing = tabsRef.current.find(t => t.path === filePath);
+      if (existing) {
+        setActiveTabId(existing.id);
+        parseHeadings(existing.content);
+      } else {
+        const title = filePath.split(/[/\\]/).pop() || 'Untitled';
+        const newTab: Tab = { id: Date.now().toString(), title, content, path: filePath };
+        setTabs(prev => [...prev, newTab]);
+        setActiveTabId(newTab.id);
+        parseHeadings(content);
       }
     }).then(fn => { unlisten = fn; });
     return () => { unlisten?.(); };
@@ -726,6 +750,7 @@ function App() {
 
   return (
     <div className={`app-container ${focusMode ? 'focus-mode' : ''}`} style={{ zoom: `${uiZoom}%` }}>
+      <a href="#main-content" className="skip-link">Skip to content</a>
       <AnimatePresence>
         {isDraggingOver && (
           <motion.div className="drop-zone" initial={{ opacity: 0 }} animate={{ opacity: 1 }} exit={{ opacity: 0 }} transition={{ duration: 0.15, ease: materialEase }}>
@@ -742,7 +767,7 @@ function App() {
 
       <AnimatePresence initial={false}>
         {!focusMode && (
-          <motion.div key="title-bar" className="title-bar" initial={{ height: 0, opacity: 0 }} animate={{ height: 32, opacity: 1 }} exit={{ height: 0, opacity: 0 }} transition={smoothTransition} style={{ overflow: 'hidden' }}>
+          <motion.header key="title-bar" className="title-bar" initial={{ height: 0, opacity: 0 }} animate={{ height: 32, opacity: 1 }} exit={{ height: 0, opacity: 0 }} transition={smoothTransition} style={{ overflow: 'hidden' }}>
             <div className="title-bar-left">
               <div className="title-bar-icon"><FileText size={14} /></div>
               <span className="title-bar-text">Vesper</span>
@@ -753,7 +778,7 @@ function App() {
               <button className="title-bar-button" onClick={() => appWindow?.toggleMaximize()}><Square size={12} /></button>
               <button className="title-bar-button close" onClick={closeWindow}><X size={14} /></button>
             </div>
-          </motion.div>
+          </motion.header>
         )}
       </AnimatePresence>
 
@@ -821,10 +846,10 @@ function App() {
       <div className="main-content">
         <AnimatePresence initial={false}>
           {showSidebar && (
-            <motion.div className="sidebar-scroll-wrapper" initial={{ width: 0, opacity: 0 }} animate={{ width: sidebarWidth, opacity: 1 }} exit={{ width: 0, opacity: 0 }} transition={resizeStartState ? { duration: 0 } : smoothTransition} style={{ height: '100%' }}>
+            <motion.nav className="sidebar-scroll-wrapper" aria-label="Table of Contents" initial={{ width: 0, opacity: 0 }} animate={{ width: sidebarWidth, opacity: 1 }} exit={{ width: 0, opacity: 0 }} transition={resizeStartState ? { duration: 0 } : smoothTransition} style={{ height: '100%' }}>
               <OverlayScrollbarsComponent options={osScrollbarOptions} style={{ height: '100%' }}>
                 <div className="sidebar" ref={sidebarRef}>
-                  <div className="sidebar-heading">Contents</div>
+                  <h2 className="sidebar-heading">Contents</h2>
                   {headings.length > 0
                     ? headings.map(h => <button key={h.id} className={`sidebar-item h${h.level}`} onClick={() => scrollToHeading(h.text)}>{h.text}</button>)
                     : <div className="sidebar-empty">No headings</div>
@@ -832,14 +857,14 @@ function App() {
                 </div>
               </OverlayScrollbarsComponent>
               <div className="sidebar-resize-handle" onMouseDown={startSidebarResize}></div>
-            </motion.div>
+            </motion.nav>
           )}
         </AnimatePresence>
 
-        <div className="content-column">
+        <main id="main-content" className="content-column">
           <AnimatePresence>
             {showSearch && activeTab && (
-              <motion.div className="search-bar" initial={{ opacity: 0, y: -10 }} animate={{ opacity: 1, y: 0 }} exit={{ opacity: 0, y: -10 }} transition={smoothTransition}>
+              <motion.search className="search-bar" initial={{ opacity: 0, y: -10 }} animate={{ opacity: 1, y: 0 }} exit={{ opacity: 0, y: -10 }} transition={smoothTransition}>
                 <input ref={searchInputRef} type="text" className="search-input" placeholder="Search..." value={searchQuery} onChange={e => { setSearchQuery(e.target.value); }} onKeyDown={e => { if (e.key === 'Enter') { e.preventDefault(); setCurrentMatchIndex(i => (i + 1) % Math.max(searchMatches.length, 1)); setSearchTriggerCount(c => c + 1); } }} autoFocus />
                 {searchMatches.length > 0 && (
                   <>
@@ -849,7 +874,7 @@ function App() {
                   </>
                 )}
                 {searchQuery && searchMatches.length === 0 && <span className="search-results">No results</span>}
-              </motion.div>
+              </motion.search>
             )}
           </AnimatePresence>
 
@@ -870,7 +895,7 @@ function App() {
               </div>
             </OverlayScrollbarsComponent>
           </motion.div>
-        </div>
+        </main>
       </div>
 
       <AnimatePresence>
diff --git a/src/styles.css b/src/styles.css
index 95144b0..d14727e 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -153,6 +153,26 @@ html, body, #root {
   overflow: visible;
 }
 
+/* Skip link — WCAG 2.4.1 */
+.skip-link {
+  position: fixed;
+  top: -100%;
+  left: 16px;
+  z-index: 99999;
+  padding: 8px 16px;
+  background-color: var(--color-accent);
+  color: white;
+  font-size: 13px;
+  font-weight: 600;
+  border-radius: var(--radius-md);
+  text-decoration: none;
+  transition: top 0.2s ease;
+}
+
+.skip-link:focus {
+  top: 8px;
+}
+
 /* App container */
 .app-container {
   display: flex;