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<h1>Combine CSS</h1>


<h2>Configuration</h2>
<p>
The 'Combine CSS' filter is enabled by specifying:
</p>
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedEnableFilters combine_css</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed EnableFilters combine_css;</pre>
</dl>
<p>
in the configuration file.
</p>

<h2>Description</h2>
<p>
'Combine CSS' seeks to reduce the number of HTTP requests made by a browser
during page refresh by replacing multiple distinct CSS files with a single CSS
file, containing the contents of all of them.  This is particularly important in
old browsers, that were limited to two connections per domain.  In addition to
reduced overhead for HTTP headers and communications warm-up, this approach
works better with TCP/IP slow-start, increasing the effective payload bit-rate
through the browser's network connection.
</p>
<p>
This <a href="/speed/docs/best-practices/rtt#CombineExternalCSS">practice</a>
reduces the number of round-trip times.
</p>

<h2>Operation</h2>
<p>
The "CSS Combine" filter finds all CSS <code>&lt;link&gt;</code> tags. If there
was more than one in a flush window, it removes each of those links and replaces
them with a single <code>&lt;link&gt;</code> to the merged document, which it
places wherever the first CSS <code>&lt;link&gt;</code> originally was.
</p>
<p>
For example, if the HTML document looks like this:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
    &lt;link rel="stylesheet" type="text/css" href="styles/yellow.css"&gt;
    &lt;link rel="stylesheet" type="text/css" href="styles/blue.css"&gt;
    &lt;link rel="stylesheet" type="text/css" href="styles/big.css"&gt;
    &lt;link rel="stylesheet" type="text/css" href="styles/bold.css"&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div class="blue yellow big bold"&gt;
      Hello, world!
    &lt;/div&gt;
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>
Then PageSpeed will rewrite it into:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
    &lt;link rel="stylesheet" type="text/css" href="styles/yellow.css+blue.css+big.css+bold.css.pagespeed.cc.xo4He3_gYf.css"&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div class="blue yellow big bold"&gt;
      Hello, world!
    &lt;/div&gt;
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<h3>Example</h3>
<p>
You can see the filter in action at <code>www.modpagespeed.com</code> on this
<a href="http://www.modpagespeed.com/combine_css.html?ModPagespeed=on&amp;ModPagespeedFilters=combine_css">example</a>.
</p>

<h2>Parameters that affect CSS optimization</h2>

<h3 id="MaxCombinedCssBytes">MaxCombinedCssBytes</h3>
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedMaxCombinedCssBytes MaxBytes</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed MaxCombinedCssBytes MaxBytes;</pre>
</dl>
<p>
<code>MaxBytes</code> is the maximum size in bytes of the combined CSS files.
CSS files larger than <code>MaxBytes</code> will be kept intact;
other CSS files will be combined into one or more files, each being no more
than <code>MaxBytes</code> in size. The current default value for
<code>MaxBytes</code> is -1 (unlimited).
</p>

<h2>Limitations</h2>
<p>The CSS Combine filter operates within the scope of a "flush window".
Specifically, large, or dynamically generated HTML files may be
"flushed" by the resource generator before they are complete.  When the
CSS combiner encounters a flush, it will emit all CSS combinations seen
up to the point of the flush.  After the flush, it will begin collecting
a new CSS combination.
</p>
<p>This filter generates URLs that are essentially the concatenation
of the URLs of all the CSS files being combined.  The maximum URL size
is generally limited to about 2k characters due to IE:
See <a href="http://support.microsoft.com/kb/208427/EN-US"
>http://support.microsoft.com/kb/208427/EN-US</a>.  Apache servers by
default impose a further limitation of about 250 characters per URL segment
(text between slashes).  PageSpeed circumvents this limitation when it runs
within Apache, but if you employ proxy servers in your path you may need to
re-impose it by overriding the setting here.  The default setting is 1024.</p>

<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedMaxSegmentLength 250</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed MaxSegmentLength 250;</pre>
</dl>

<h3 id="permit-ids-for-css-combining">Combining Resources with IDs</h3>
<p class="note"><strong>Note: New feature as of 1.12.34.1</strong></p>

<p>
  By default PageSpeed won't combine CSS files that have <code>id</code>
  attributes, because this often indicates that the site designer intended to
  reference them in javascript.  However, some content management systems,
  including WordPress, put <code>id</code>s on all stylesheets for clarity.  To
  enable combining these files, you can provide one or more wildcards.  For
  example, this would mark stylesheets with ids starting with <code>font</code>
  as eligible for combining:
</p>
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedPermitIdsForCssCombining font*</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed PermitIdsForCssCombining font*;</pre>
</dl>

<h2>Requirements</h2>
<p>
The 'Combine CSS' filter may need to <em>absolutify</em> relative
URLs, if rewriting the CSS causes the path to be moved.  The filter
will not merge together resources from multiple distinct domains, even
if those domains are each authorized by <code>Domain</code>.
It <strong>will</strong> merge together resources from multiple
distinct domains that have been mapped together via
<code>MapRewriteDomain</code>.
</p>
<p>
By default, the filter will combine together CSS files from different
paths, placing the combined element at the lowest level common to both
origins.  In some cases, this may be undesirable.  You can turn off the
behavior with:
</p>
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedCombineAcrossPaths off</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed CombineAcrossPaths off;</pre>
</dl>
<p>
The filter will maintain the order of the CSS contents, as class order can be
significant.
</p>
<p>
IE Directives containing CSS links form a "barrier" for the CSS combiner.
Multiple CSS elements found before an IE directive are combined together
immediately before the IE directive.  Multiple CSS elements found after are
also combined, but the combination does not span across the IE directive, as
that would affect the order that the browser sees the CSS elements.
</p>

<h2>Risks</h2>
<p>
This filter is considered low risk. However, JavaScript can be written that
walks the DOM looking for <code>&lt;link&gt;</code> entries with certain
syntax.  Such JavaScript may behave differently on a page which has modified
CSS links in this way.
</p>


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