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 <section id="technical-overview">
 <span id="overview"></span><h1 id="technical-overview"><span id="overview"></span>Technical Overview</h1>
 <div class="contents local" id="contents" style="display: none">
 <ul class="small-gap">
 <li><a class="reference internal" href="#introduction" id="id2">Introduction</a></li>
 <li><a class="reference internal" href="#why-use-native-client" id="id3">Why use Native Client?</a></li>
 <li><a class="reference internal" href="#common-use-cases" id="id4">Common use cases</a></li>
 <li><p class="first"><a class="reference internal" href="#how-native-client-works" id="id5">How Native Client works</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#security" id="id6">Security</a></li>
 <li><a class="reference internal" href="#portability" id="id7">Portability</a></li>
 <li><a class="reference internal" href="#toolchains" id="id8">Toolchains</a></li>
 </ul>
 </li>
 <li><p class="first"><a class="reference internal" href="#native-client-in-a-web-application" id="id9">Native Client in a web application</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#pepper-plugin-api" id="id10">Pepper Plugin API</a></li>
 </ul>
 </li>
 <li><a class="reference internal" href="#versioning" id="id11">Versioning</a></li>
 <li><a class="reference internal" href="#where-to-start" id="id12">Where to start</a></li>
 </ul>
 
-</div><section id="introduction">
-<h2 id="introduction">Introduction</h2>
+</div><h2 id="introduction">Introduction</h2>
 <p><strong>Native Client</strong> (NaCl) is an open-source technology for running native
 compiled code in the browser, with the goal of maintaining the portability
 and safety that users expect from web applications. Native Client expands web
 programming beyond JavaScript, enabling developers to enhance their web
 applications using their preferred language. This document describes some of
 the key benefits and common use cases of Native Client.</p>
 <p>Google has implemented the open-source <a class="reference external" href="http://www.chromium.org/nativeclient">Native Client project</a> in the Chrome browser on Windows, Mac,
 Linux, and Chrome OS. The <a class="reference internal" href="/native-client/sdk/download.html"><em>Native Client Software Development Kit (SDK)</em></a>, itself an open-source project, lets developers create web
 applications that use NaCl and run in Chrome across multiple platforms.</p>
 <p>A web application that uses Native Client generally consists of a combination of
 JavaScript, HTML, CSS, and a NaCl module that is written in a language supported
 by the SDK. The NaCl SDK currently supports C and C++; as compilers for
 additional languages are developed, the SDK will be updated to support those
 languages as well.</p>
 <img alt="/native-client/images/web-app-with-nacl.png" src="/native-client/images/web-app-with-nacl.png" />
-</section><section id="why-use-native-client">
 <h2 id="why-use-native-client">Why use Native Client?</h2>
 <p>Native Client open-source technology is designed to run compiled code
 securely inside a browser at near-native speeds. Native Client puts web
 applications on the same playing field as traditional (locally-run)
 software&#8212;it provides the means to fully harness the client&#8217;s computational
 resources for applications such as 3D games, multimedia editors, CAD modeling,
 client-side data analytics, and interactive simulations.
 Native Client also aims to give C and C++ (and eventually other languages) the
 same level of portability and safety that JavaScript provides on the web today.</p>
 <p>Here are a few of the key benefits that Native Client offers:</p>
@@ skipping 12 matching lines @@
 <li><strong>Security:</strong> Native Client uses a double sandbox model designed to protect
 the user&#8217;s system from malicious or buggy applications. This model offers the
 safety of traditional web applications without sacrificing performance and
 without requiring users to install a plugin.</li>
 <li><strong>Performance:</strong> Native Client allows web applications to run at speeds
 comparable to desktop applications (within 5-15% of native speed).
 Native Client also allows applications to harness all available CPU cores via
 a threading API; this enables demanding applications such as console-quality
 games to run inside the browser.</li>
 </ul>
-</section><section id="common-use-cases">
 <h2 id="common-use-cases">Common use cases</h2>
 <p>Typical use cases for Native Client include the following:</p>
 <ul class="small-gap">
 <li><strong>Existing software components:</strong> With support for C and C++, Native
 Client enables you to reuse existing software modules in
 web applications&#8212;you don&#8217;t need to rewrite and debug code
 that&#8217;s already proven to work well.</li>
 <li><strong>Legacy desktop applications:</strong> Native Client provides a smooth migration
 path from desktop applications to the web. You can port and recompile existing
 code for the computation engine of your application directly to Native Client,
@@ skipping 14 matching lines @@
 access low-latency audio, networking APIs, and OpenGL ES with programmable
 shaders. Native Client is a natural fit for running a physics engine or
 artificial intelligence module that powers a sophisticated web game.
 Native Client also enables applications to run unchanged across
 many platforms.</li>
 <li><strong>Any application that requires acceleration</strong>: Native Client fits seamlessly
 into web applications&#8212;it&#8217;s up to you to decide to what extent to use it.
 Use of Native Client covers the full spectrum from complete applications to
 small optimized routines that accelerate vital parts of web apps.</li>
 </ul>
-</section><section id="how-native-client-works">
-<span id="link-how-nacl-works"></span><h2 id="how-native-client-works"><span id="link-how-nacl-works"></span>How Native Client works</h2>
+<h2 id="how-native-client-works"><span id="link-how-nacl-works"></span>How Native Client works</h2>
 <p>Native Client is an umbrella name for a set of interrelated software components
 that work together to provide a way to develop C/C++ applications and run them
 securely on the web.</p>
 <p>At a high level, Native Client consists of:</p>
 <ul class="small-gap">
 <li><strong>Toolchains</strong>: collections of development tools (compilers, linkers, etc.)
 that transform C/C++ code to Native Client modules.</li>
 <li><strong>Runtime components</strong>: components embedded in the browser or other
 host platforms that allow execution of  Native Client modules
 securely and efficiently.</li>
 </ul>
 <p>The following diagram shows how these components interact:</p>
 <img alt="/native-client/images/nacl-pnacl-component-diagram.png" src="/native-client/images/nacl-pnacl-component-diagram.png" />
 <p>The left side of the diagram shows how to use Portable Native Client
 (PNaCl, pronounced &#8220;pinnacle&#8221;). Developers use the PNaCl toolchain
 to produce a single, portable (<strong>pexe</strong>) module. At runtime, a translator
 built into the browser translates the pexe into native code for the
 relevant client architecture.</p>
 <p>The right side of the diagram shows how to use traditional (non-portable)
 Native Client. Developers use a nacl-gcc based toolchain to produce multiple
 architecture-dependent (<strong>nexe</strong>) modules, which are packaged into an
 application. At runtime, the browser decides which nexe to load based
 on the architecture of the client machine.</p>
-<section id="security">
 <h3 id="security">Security</h3>
 <p>Since Native Client permits the execution of native code on client machines,
 special security measures have to be implemented:</p>
 <ul class="small-gap">
 <li>The NaCl sandbox ensures that code accesses system resources only through
 safe, whitelisted APIs, and operates within its limits without attempting to
 interfere with other code running either within the browser or outside it.</li>
 <li>The NaCl validator statically analyzes code prior to running it
 to make sure it only uses code and data patterns that are permitted and safe.</li>
 </ul>
 <p>The above security measures are in addition to the existing sandbox in the
 Chrome browser&#8212;the Native Client module always executes in a process with
 restricted permissions. The only interaction between this process and the
 outside world is through sanctioned browser interfaces. Because of the
 combination of the NaCl sandbox and the Chrome sandbox, we say that
 Native Client employs a double sandbox design.</p>
-</section><section id="portability">
 <h3 id="portability">Portability</h3>
 <p>Portable Native Client (PNaCl, prounounced &#8220;pinnacle&#8221;) employs state-of-the-art
 compiler technology to compile C/C++ source code to a portable bitcode
 executable (<strong>pexe</strong>). PNaCl bitcode is an OS- and architecture-independent
 format that can be freely distributed on the web and <a class="reference internal" href="#link-nacl-in-web-apps"><em>embedded in web
 applications</em></a>.</p>
 <p>The PNaCl translator is a component embedded in the Chrome browser; its task is
 to run pexe modules. Internally, the translator compiles a pexe to a nexe
 (a native executable for the client platform&#8217;s architecture), and then executes
 the nexe within the Native Client sandbox as described above. It also uses
 intelligent caching to avoid re-compiling the pexe if it was previously compiled
 on the client&#8217;s browser.</p>
 <p>Native Client also supports the execution of nexe modules directly in the
 browser. However, since nexes contain architecture-specific machine code,
 they are not allowed to be distributed on the open web&#8212;they can only be
 used as part of applications and extensions that are installed from the
 Chrome Web Store.</p>
 <p>For more details on the difference between NaCl and PNaCl, see
 <a class="reference internal" href="/native-client/nacl-and-pnacl.html"><em>NaCl and PNaCl</em></a>.</p>
-</section><section id="toolchains">
-<span id="id1"></span><h3 id="toolchains"><span id="id1"></span>Toolchains</h3>
+<h3 id="toolchains"><span id="id1"></span>Toolchains</h3>
 <p>A toolchain is a set of tools used to create an application from a set of
 source files. In the case of Native Client, a toolchain consists of a compiler,
 linker, assembler and other tools that are used to convert an
 application written in C/C++ into a module that is loadable by the browser.</p>
 <p>The Native Client SDK provides two toolchains:</p>
 <ul class="small-gap">
 <li>a <strong>PNaCl toolchain</strong> for generating portable NaCl modules (pexe files)</li>
 <li>a <strong>gcc-based toolchain (nacl-gcc)</strong> for generating non-portable NaCl modules
 (nexe files)</li>
 </ul>
 <p>The PNaCl toolchain is recommended for most applications. The nacl-gcc
 toolchain should only be used for applications that will not be distributed
 on the open web.</p>
-</section></section><section id="native-client-in-a-web-application">
-<span id="link-nacl-in-web-apps"></span><h2 id="native-client-in-a-web-application"><span id="link-nacl-in-web-apps"></span>Native Client in a web application</h2>
+<h2 id="native-client-in-a-web-application"><span id="link-nacl-in-web-apps"></span>Native Client in a web application</h2>
 <p id="application-files">A Native Client application consists of a set of files:</p>
 <ul class="small-gap">
 <li><strong>HTML</strong>, <strong>CSS</strong>, and <strong>JavaScript</strong> files, as in any modern web
 application. The JavaScript code is responsible for communicating with the
 NaCl module.</li>
 <li>A <strong>pexe</strong> (portable NaCl) file. This module uses the <a class="reference internal" href="#link-pepper"><em>Pepper</em></a> API, which provides the bridge to JavaScript and
 browser resources.</li>
 <li>A Native Client <strong>manifest</strong> file that specifies the pexe to load, along with
 some loading options. This manifest file is embedded into the HTML page
 through an <code>&lt;embed&gt;</code> tag, as shown in the figure below.</li>
 </ul>
 <img alt="/native-client/images/nacl-in-a-web-app.png" src="/native-client/images/nacl-in-a-web-app.png" />
 <p>For more details, see <a class="reference internal" href="/native-client/devguide/coding/application-structure.html"><em>Application Structure</em></a>.</p>
-<section id="pepper-plugin-api">
-<span id="link-pepper"></span><h3 id="pepper-plugin-api"><span id="link-pepper"></span>Pepper Plugin API</h3>
+<h3 id="pepper-plugin-api"><span id="link-pepper"></span>Pepper Plugin API</h3>
 <p>The Pepper Plugin API (PPAPI), called <strong>Pepper</strong> for convenience, is an
 open-source, cross-platform C/C++ API for web browser plugins. From the point
 of view of Native Client, Pepper allows a C/C++ module to communicate with
 the hosting browser and get access to system-level functions in a safe and
 portable way. One of the security constraints in Native Client is that modules
 cannot make any OS-level calls directly. Pepper provides analogous APIs that
 modules can target instead.</p>
 <p>You can use the Pepper APIs to gain access to the full array of browser
 capabilities, including:</p>
 <ul class="small-gap">
 <li><a class="reference internal" href="/native-client/devguide/coding/message-system.html"><em>Talking to the JavaScript code in your application</em></a> from the C++ code in your NaCl module.</li>
 <li><a class="reference internal" href="/native-client/devguide/coding/file-io.html"><em>Doing file I/O</em></a>.</li>
 <li><a class="reference internal" href="/native-client/devguide/coding/audio.html"><em>Playing audio</em></a>.</li>
 <li><a class="reference internal" href="/native-client/devguide/coding/3D-graphics.html"><em>Rendering 3D graphics</em></a>.</li>
 </ul>
 <p>Pepper includes both a C API and a C++ API. The C++ API is a set of bindings
 written on top of the C API. For additional information about Pepper, see
 <a class="reference external" href="http://code.google.com/p/ppapi/wiki/Concepts">Pepper Concepts</a>.</p>
-</section></section><section id="versioning">
 <h2 id="versioning">Versioning</h2>
 <p>Chrome is released on a six week cycle, and developer versions of Chrome are
 pushed to the public beta channel three weeks before each release. As with any
 software, each release of Chrome may include changes to Native Client and the
 Pepper interfaces that may require modification to existing applications.
 However, modules compiled for one version of Pepper/Chrome should work with
 subsequent versions of Pepper/Chrome. The SDK includes multiple versions of the
 Pepper APIs to help developers make adjustments to API changes and take
 advantage of new features: <a class="reference external" href="/native-client/pepper_stable">stable</a>, <a class="reference external" href="/native-client/pepper_beta">beta</a> and <a class="reference external" href="/native-client/pepper_dev">dev</a>.</p>
-</section><section id="where-to-start">
 <h2 id="where-to-start">Where to start</h2>
 <p>The <a class="reference internal" href="/native-client/quick-start.html"><em>Quick Start</em></a> document provides links to downloads and
 documentation that should help you get started with developing and distributing
 Native Client applications.</p>
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