guide/webapps/targeting.jd

page.title=Supporting Different Screens in Web Apps
@jd:body

<div id="qv-wrapper">
<div id="qv">
<h2>In this document</h2>
<ol>
<li><a href="#Metadata">Using Viewport Metadata</a>
  <ol>
    <li><a href="#ViewportSize">Defining the viewport size</a></li>
    <li><a href="#ViewportScale">Defining the viewport scale</a></li>
    <li><a href="#ViewportDensity">Defining the viewport target density</a></li>
  </ol>
</li>
<li><a href="#DensityCSS">Targeting Device Density with CSS</a></li>
<li><a href="#DensityJS">targeting Device Density with JavaScript</a></li>
</ol>

<h2>See also</h2>
<ul>
  <li><a href="https://developers.google.com/chrome/mobile/docs/webview/pixelperfect"
  >Pixel-Perfect UI in the WebView</a></li>
  <li><a href="http://www.html5rocks.com/en/mobile/responsivedesign/" class="external-link">Creating
  a Mobile-First Responsive Web Design</a></li>
  <li><a href="http://www.html5rocks.com/en/mobile/high-dpi/" class="external-link">High
  DPI Images for Variable Pixel Densities</a></li>
</ul>

</div>
</div>


<p>Because Android is available on devices with a variety of screen sizes and pixel densities, you
should account for these factors in your web design so your web pages always appear at the
appropriate size.</p>


<p>When targeting your web pages for Android devices, there are two major factors that you
should account for:</p>

<dl>
  <dt><a href="#Viewport">The viewport</a></dt>
    <dd>The viewport is the rectangular area that provides a drawable region for your web page.
    You can specify several viewport properties, such as its size and initial scale. Most important
    is the view port width, which defines the total number of horizontal pixels available from the web page's point of
    view (the number of CSS pixels available).
   </dd>

  <dt><a href="#DensityCSS">The screen density</a></dt>
<dd>The {@link android.webkit.WebView} class and most web browsers on Android convert your CSS
pixel values to density-independent pixel values, so your web page appears at the same perceivable
size as a medium-density screen (about 160dpi). However, if graphics are an important element of
your web design, you should pay close attention to the scaling that occurs on different densities,
because a 300px-wide image on a 320dpi screen will be scaled up (using more physical pixels per CSS
pixel), which can produce artifacts (blurring and pixelation).</dd>

</dl>


<h2 id="Viewport">Specifying Viewport Properties</h2>

<p>The viewport is the area in which your web page is drawn. Although the viewport's total visible
area matches the size of the screen when zoomed all the way out, the viewport has its own pixel
dimensions that it makes available to a web page. For example, although a device screen might
have physical a width
of 480 pixels, the viewport can have a width of 800 pixels. This allows a web page designed at 800
pixels wide to be completely visible on the screen when the viewport scale is 1.0. Most web browsers on
Android (including Chrome) set the viewport to a large size by default (known as "wide viewport
mode" at about 980px wide). Many browsers also zoom out as far as possible, by default, to
show the full viewport width (known as "overview mode").</p>

<p class="note"><strong>Note:</strong>
When your page is rendered in a {@link android.webkit.WebView}, it does not use wide viewport mode
(the page appears at full zoom) by default. You can enable wide viewport mode
with {@link android.webkit.WebSettings#setUseWideViewPort setUseWideViewPort()}.</p>

<p>You can define properties of the viewport for your web page, such as the width and initial zoom
level, using the {@code <meta name="viewport" ...>} tag in your document
{@code <head>}.</p>

<p>The following syntax shows all of the
supported viewport properties and the types of values accepted by each one:</p>

<pre>
&lt;meta name="viewport"
      content="
          <b>height</b> = [<em>pixel_value</em> | "device-height"] ,
          <b>width</b> = [<em>pixel_value</em> | "device-width"] ,
          <b>initial-scale</b> = <em>float_value</em> ,
          <b>minimum-scale</b> = <em>float_value</em> ,
          <b>maximum-scale</b> = <em>float_value</em> ,
          <b>user-scalable</b> = ["yes" | "no"]
          " /&gt;
</pre>

<p>For example, the following {@code <meta>} tag specifies that the viewport width
should exactly match the device screen's width and that the ability to zoom should be disabled:</p>

<pre>
&lt;head&gt;
    &lt;title&gt;Example&lt;/title&gt;
    &lt;meta name="viewport" content="width=device-width, user-scalable=no" /&gt;
&lt;/head&gt;
</pre>


<p>When optimizing your site for mobile devices, you should usually set the width to
{@code "device-width"} so the size fits exactly on all devices, then use CSS media queries to
flexibly adapt layouts to suit different screen sizes.

<p class="note"><strong>Note:</strong> You should disable user scaling only when you're certain
that your web page layout is flexible and the content will fit the width of small screens.</p>


<h2 id="DensityCSS">Targeting Device Density with CSS</h2>

<p>The Android Browser and {@link android.webkit.WebView} support a CSS media feature that allows
you to create styles for specific
screen densities&mdash;the <code>-webkit-device-pixel-ratio</code> CSS media feature. The
value you apply to this feature should be either
"0.75", "1", or "1.5", to indicate that the styles are for devices with low density, medium density,
or high density screens, respectively.</p>

<p>For example, you can create separate stylesheets for each density:</p>

<pre>
&lt;link rel="stylesheet" media="screen and (-webkit-device-pixel-ratio: 1.5)" href="hdpi.css" /&gt;
&lt;link rel="stylesheet" media="screen and (-webkit-device-pixel-ratio: 1.0)" href="mdpi.css" /&gt;
</pre>


<p>Or, specify the different styles in one stylesheet:</p>

<pre class="no-pretty-print">
#header {
    background:url(medium-density-image.png);
}

&#64;media screen and (-webkit-device-pixel-ratio: 1.5) {
    /* CSS for high-density screens */
    #header {
        background:url(high-density-image.png);
    }
}

&#64;media screen and (-webkit-device-pixel-ratio: 0.75) {
    /* CSS for low-density screens */
    #header {
        background:url(low-density-image.png);
    }
}
</pre>


<p>For more information about handling different screen densities, especially images, see
<a href="http://www.html5rocks.com/en/mobile/high-dpi/" class="external-link">High
  DPI Images for Variable Pixel Densities</a>.</li>


<h2 id="DensityJS">Targeting Device Density with JavaScript</h2>

<p>The Android Browser and {@link android.webkit.WebView} support a DOM property that allows you to
query the density of the current
device&mdash;the <code>window.devicePixelRatio</code> DOM property. The value of this property
specifies the scaling factor used for the current device. For example, if the value
of <code>window.devicePixelRatio</code> is "1.0", then the device is considered a medium density
device and no scaling is applied by default; if the value is "1.5", then the device is
considered a high density device and the page is scaled 1.5x by default; if the value
is "0.75", then the device is considered a low density device and the page is scaled
0.75x by default. Of course, the scaling that the Android Browser and {@link android.webkit.WebView}
apply is based on the web page's
target density&mdash;as described in the section about <a href="#ViewportDensity">Defining the
viewport target density</a>, the default target is medium-density, but you can change the
target to affect how your web page is scaled for different screen densities.</p>

<p>For example, here's how you can query the device density with JavaScript:</p>

<pre>
if (window.devicePixelRatio == 1.5) {
  alert("This is a high-density screen");
} else if (window.devicePixelRatio == 0.75) {
  alert("This is a low-density screen");
}
</pre>