chrome/common/extensions/docs/templates/intros/browserAction.html

<p>
In the following figure,
the multicolored square
to the right of the address bar
is the icon for a browser action.
A popup is below the icon.
</p>

<img src="{{static}}/images/browser-action.png"
  width="363" height="226" />

<p>
If you want to create an icon that isn't always visible,
use a <a href="pageAction.html">page action</a>
instead of a browser action.
</p>

<h2 id="manifest">Manifest</h2>

<p>
Register your browser action in the
<a href="manifest.html">extension manifest</a>
like this:
</p>

<pre>{
  "name": "My extension",
  ...
  <b>"browser_action": {
    "default_icon": {                    <em>// optional</em>
      "19": "images/icon19.png",           <em>// optional</em>
      "38": "images/icon38.png"            <em>// optional</em>
    },
    "default_title": "Google Mail",      <em>// optional; shown in tooltip</em>
    "default_popup": "popup.html"        <em>// optional</em>
  }</b>,
  ...
}</pre>

<p>
If you only provide one of the 19px or 38px icon size, the extension system will
scale the icon you provide to the pixel density of the user's display, which
can lose detail or make it look fuzzy. The old syntax for registering the
default icon is still supported:
</p>

<pre>{
  "name": "My extension",
  ...
  <b>"browser_action": {
    ...
    "default_icon": "images/icon19.png"  <em>// optional</em>
    <em>// equivalent to "default_icon": { "19": "images/icon19.png" }</em>
  }</b>,
  ...
}</pre>

<h2 id="ui">Parts of the UI</h2>

<p>
A browser action can have an <a href="#icon">icon</a>,
a <a href="#tooltip">tooltip</a>,
a <a href="#badge">badge</a>,
and a <a href="#popups">popup</a>.
</p>

<h3 id="icon">Icon</h3>

<p>Browser action icons can be up to 19 dips (device-independent pixels)
  wide and high. Larger icons are resized to fit, but for best results,
  use a 19-dip square icon.</p>

<p>You can set the icon in two ways:
  using a static image or using the
  HTML5 <a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html">canvas element</a>.
  Using static images is easier for simple applications,
  but you can create more dynamic UIs &mdash;
  such as smooth animation &mdash;
  using the canvas element.
  </p>

<p>Static images can be in any format WebKit can display,
  including BMP, GIF, ICO, JPEG, or PNG.
  </p>

<p>To set the icon,
  use the <b>default_icon</b> field of <b>browser_action</b>
  in the <a href="#manifest">manifest</a>,
  or call the $ref:browserAction.setIcon method.
  </p>

<p>To properly display icon when screen pixel density (ratio
  <code>size_in_pixel / size_in_dip</code>) is different than 1,
  the icon can be defined as set of images with different sizes.
  The actual image to display will be selected from the set to
  best fit the pixel size of 19 dip icon.
  At the moment, the icon set can contain images with pixel sizes 19 and 38.
  </p>

<h3 id="tooltip">Tooltip</h3>

<p>
To set the tooltip,
use the <b>default_title</b> field of <b>browser_action</b>
in the <a href="#manifest">manifest</a>,
or call the $ref:browserAction.setTitle method.
You can specify locale-specific strings for the <b>default_title</b> field;
see <a href="i18n.html">Internationalization</a> for details.
</p>

<h3 id="badge">Badge</h3>

<p>Browser actions can optionally display a <em>badge</em> &mdash;
  a bit of text that is layered over the icon.
  Badges make it easy to update the browser action
  to display a small amount of information
  about the state of the extension.</p>

<p>Because the badge has limited space,
   it should have 4 characters or less.
  </p>

<p>
Set the text and color of the badge using
$ref:browserAction.setBadgeText and
$ref:browserAction.setBadgeBackgroundColor,
respectively.

</p>


<h3 id="popups">Popup</h3>

<p>If a browser action has a popup,
  the popup appears when the user clicks the icon.
  The popup can contain any HTML contents that you like,
  and it's automatically sized to fit its contents.
  </p>

<p>
To add a popup to your browser action,
create an HTML file with the popup's contents.
Specify the HTML file in the <b>default_popup</b> field of <b>browser_action</b>
in the <a href="#manifest">manifest</a>, or call the
$ref:browserAction.setPopup method.
</p>

<h2 id="tips">Tips</h2>

<p>For the best visual impact,
  follow these guidelines:</p>

<ul>
  <li><b>Do</b> use browser actions for features
    that make sense on most pages.
  <li><b>Don't</b> use browser actions for features
    that make sense for only a few pages.
    Use <a href="pageAction.html">page actions</a> instead.
  <li><b>Do</b> use big, colorful icons that make the most of
    the 19x19-pixel space.
    Browser action icons should seem a little bigger
    and heavier than page action icons.
  <li><b>Don't</b> attempt to mimic
    Google Chrome's monochrome menu icon.
    That doesn't work well with themes, and anyway,
    extensions should stand out a little.
  <li><b>Do</b> use alpha transparency
    to add soft edges to your icon.
    Because many people use themes,
    your icon should look nice
    on a variety of background colors.
  <li><b>Don't</b> constantly animate your icon.
    That's just annoying.
</ul>

<h2 id="examples"> Examples </h2>

<p>
You can find simple examples of using browser actions in the
<a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/browserAction/">examples/api/browserAction</a>
directory.
For other examples and for help in viewing the source code, see
<a href="samples.html">Samples</a>.
</p>