doc/filter-attribute-elide.html - incubator-pagespeed-website - Git at Google

 <!--
 Licensed to the Apache Software Foundation (ASF) under one
 or more contributor license agreements.  See the NOTICE file
 distributed with this work for additional information
 regarding copyright ownership.  The ASF licenses this file
 to you under the Apache License, Version 2.0 (the
 "License"); you may not use this file except in compliance
 with the License.  You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing,
 software distributed under the License is distributed on an
 "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 KIND, either express or implied.  See the License for the
 specific language governing permissions and limitations
 under the License.
 -->

 <html>
   <head>
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>Elide Attributes</title>
     <link rel="stylesheet" href="doc.css">
   </head>
   <body>
 <!--#include virtual="_header.html" -->


   <div id=content>
 <h1>Elide Attributes</h1>


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

 <h2>Description</h2>
 <p>
 The 'Elide Attributes' filter reduces the transfer size of HTML files
 by removing attributes from tags when the specified value is equal to
 the default value for that attribute. This can save a modest number of
 bytes, and may make the document more compressible by canonicalizing
 the affected tags.
 </p>

 <h2>Operation</h2>
 <p>
 There are two cases where an attribute value can be removed.  First,
 some attributes are "single-valued" or "boolean", in that the value
 specified for the attribute is irrelevant -- all that matters is
 whether the attribute is present or not.  In such cases, the filter
 will remove the value from the tag, leaving only the attribute name.
 </p>
 <p>
 For example, the following tag:
 </p>
 <pre class="prettyprint">
   &lt;button name="ok" disabled="disabled"&gt;
 </pre>
 <p>
 can be rewritten to:
 </p>
 <pre class="prettyprint">
   &lt;button name="ok" disabled&gt;
 </pre>
 <p>
 The second case is an optional attribute with a default value.  If an
 HTML attribute includes an explicit value for an attribute (perhaps to
 aid readability) that is equal to the default attribute, the filter
 will remove the attribute name and value, knowing that the browser
 will infer the intended attribute anyway.  For example, the following
 tag:
 </p>
 <pre class="prettyprint">
   &lt;form method=get&gt;
 </pre>
 <p>
 can be rewritten to:
 </p>
 <pre class="prettyprint">
   &lt;form&gt;
 </pre>
 <h3>Example</h3>
 <p>
 You can see the filter in action at <code>www.modpagespeed.com</code> on this
 <a href="https://www.modpagespeed.com/examples/elide_attributes.html?ModPagespeed=on&amp;ModPagespeedFilters=elide_attributes">example</a>.
 </p>

 <h2>Requirements</h2>
 <p>
 The 'Elide Attributes' filter must be wary of documents with an XHTML
 doctype, as removing the value from a single-valued attribute will
 result in invalid XHTML.  The filter attempts to recognize XHTML
 doctype declarations and will disable this rewriting feature in such
 cases.
 </p>

 <h2>Risks</h2>
 <p>This filter is considered medium risk. It is safe for most pages, but
   might break CSS formatting on certain pages that match on default attributes
   that this filter removes. Further, JavaScript can be written that inspects
   the DOM looking for the presence of certain attributes. Such JavaScript may
   behave differently on a page which has removed otherwise unnecessary
   attributes.</p>


 </div>
   <!--#include virtual="_footer.html" -->
   </body>
 </html>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one
	or more contributor license agreements. See the NOTICE file
	distributed with this work for additional information
	regarding copyright ownership. The ASF licenses this file
	to you under the Apache License, Version 2.0 (the
	"License"); you may not use this file except in compliance
	with the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing,
	software distributed under the License is distributed on an
	"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	KIND, either express or implied. See the License for the
	specific language governing permissions and limitations
	under the License.
	-->

	<html>
	<head>
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title>Elide Attributes</title>
	<link rel="stylesheet" href="doc.css">
	</head>
	<body>
	<!--#include virtual="_header.html" -->


	<div id=content>
	<h1>Elide Attributes</h1>


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

	<h2>Description</h2>
	<p>
	The 'Elide Attributes' filter reduces the transfer size of HTML files
	by removing attributes from tags when the specified value is equal to
	the default value for that attribute. This can save a modest number of
	bytes, and may make the document more compressible by canonicalizing
	the affected tags.
	</p>

	<h2>Operation</h2>
	<p>
	There are two cases where an attribute value can be removed. First,
	some attributes are "single-valued" or "boolean", in that the value
	specified for the attribute is irrelevant -- all that matters is
	whether the attribute is present or not. In such cases, the filter
	will remove the value from the tag, leaving only the attribute name.
	</p>
	<p>
	For example, the following tag:
	</p>
	<pre class="prettyprint">
	<button name="ok" disabled="disabled">
	</pre>
	<p>
	can be rewritten to:
	</p>
	<pre class="prettyprint">
	<button name="ok" disabled>
	</pre>
	<p>
	The second case is an optional attribute with a default value. If an
	HTML attribute includes an explicit value for an attribute (perhaps to
	aid readability) that is equal to the default attribute, the filter
	will remove the attribute name and value, knowing that the browser
	will infer the intended attribute anyway. For example, the following
	tag:
	</p>
	<pre class="prettyprint">
	<form method=get>
	</pre>
	<p>
	can be rewritten to:
	</p>
	<pre class="prettyprint">
	<form>
	</pre>
	<h3>Example</h3>
	<p>
	You can see the filter in action at <code>www.modpagespeed.com</code> on this
	<a href="https://www.modpagespeed.com/examples/elide_attributes.html?ModPagespeed=on&ModPagespeedFilters=elide_attributes">example</a>.
	</p>

	<h2>Requirements</h2>
	<p>
	The 'Elide Attributes' filter must be wary of documents with an XHTML
	doctype, as removing the value from a single-valued attribute will
	result in invalid XHTML. The filter attempts to recognize XHTML
	doctype declarations and will disable this rewriting feature in such
	cases.
	</p>

	<h2>Risks</h2>
	<p>This filter is considered medium risk. It is safe for most pages, but
	might break CSS formatting on certain pages that match on default attributes
	that this filter removes. Further, JavaScript can be written that inspects
	the DOM looking for the presence of certain attributes. Such JavaScript may
	behave differently on a page which has removed otherwise unnecessary
	attributes.</p>


	</div>
	<!--#include virtual="_footer.html" -->
	</body>
	</html>