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| |
| <html> |
| <head> |
| <meta name="viewport" content="width=device-width, initial-scale=1"> |
| <title>Move CSS to Head</title> |
| <link rel="stylesheet" href="doc.css"> |
| </head> |
| <body> |
| <!--#include virtual="_header.html" --> |
| |
| |
| <div id=content> |
| <h1>Move CSS to Head</h1> |
| |
| |
| <h2>Configuration</h2> |
| <p> |
| The 'Move CSS to head' filter is enabled by specifying: |
| </p> |
| <dl> |
| <dt>Apache:<dd><pre class="prettyprint" |
| >ModPagespeedEnableFilters move_css_to_head</pre> |
| <dt>Nginx:<dd><pre class="prettyprint" |
| >pagespeed EnableFilters move_css_to_head;</pre> |
| </dl> |
| <p> |
| in the configuration file. |
| </p> |
| |
| <h2>Description</h2> |
| <p> |
| 'Move CSS to head' seeks to reduce the number of times the browser must |
| re-flow the document by ensuring that the CSS styles are all parsed in |
| the head, before any body elements are introduced. |
| </p> |
| <p> |
| This is based on the |
| <a target="_blank" href="https://developers.google.com/speed/docs/best-practices/rendering#PutCSSInHead"> |
| best practice for optimizing browser rendering. |
| </a> |
| </p> |
| |
| <h2>Operation</h2> |
| <p> |
| The 'Move CSS to head' filter operates only on CSS |
| <code><link</code> and <code><style></code> tags found after the |
| <code></head></code> and moves these links back into the |
| <code><head></code> ... <code></head></code> section. |
| </p> |
| <p> |
| For example, if the HTML document looks like this: |
| </p> |
| <pre class="prettyprint"> |
| <html> |
| <head> |
| </head> |
| <body> |
| <script src="script.js" type="text/javascript"></script> |
| <div class="blue yellow big bold"> |
| Hello, world! |
| </div> |
| <style type="text/css"> |
| .foo { color: red; } |
| </style> |
| <link rel="stylesheet" type="text/css" href="styles/all_styles.css"> |
| </body> |
| </html> |
| </pre> |
| <p> |
| Then PageSpeed will rewrite it into: |
| </p> |
| <pre class="prettyprint"> |
| <html> |
| <head> |
| <style type="text/css"> |
| .foo { color: red; } |
| </style> |
| <link rel="stylesheet" type="text/css" href="styles/all_styles.css"> |
| </head> |
| <body> |
| <script src="script.js" type="text/javascript"></script> |
| <div class="blue yellow big bold"> |
| Hello, world! |
| </div> |
| </body> |
| </html> |
| </pre> |
| |
| <p> |
| In some browsers, the original version will flash quickly as the |
| browser will render the "Hello, world!" text before it sees the style |
| tags providing definitions for the CSS classes. This transformation |
| will eliminate that flash, but the end result will be the same. |
| </p> |
| |
| <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/move_css_to_head.html?ModPagespeed=on&ModPagespeedFilters=move_css_to_head">example</a>. |
| </p> |
| |
| <h2>Limitations</h2> |
| <p> |
| This 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. If the filter |
| encounters a flush after the end of the <code><head></code> section, |
| subsequently encountered CSS elements will not be moved into the |
| <code><head></code> section. |
| </p> |
| |
| <h2>Risks</h2> |
| <p> |
| This filter is considered low risk. However, JavaScript that is |
| executed before a CSS element will see a different view of the DOM in |
| the presence of this rewriter: the CSS element will now be in the head. |
| If there is such JavaScript embedded in the middle of a page then this |
| rewriter may change its behavior. |
| </p> |
| |
| |
| </div> |
| <!--#include virtual="_footer.html" --> |
| </body> |
| </html> |
