META Refresh Nullifies Caching for IE6 and IE7

March 8, 2010

There has been some interesting discussion recently on the mailing list for Google’s Page Speed performance tool. Brian Brophy rediscovered a critical performance bug in Internet Explorer that Joseph Smarr had found nearly 3 years ago. Both Internet Explorer 6 and 7 are affected by this bug . IE8 is not affected.

To summarize, the bug is this: When a site uses a <META> refresh tag to send the visitor to a URL, IE6 and IE7 treat that as if the user had clicked the “Refresh” or “Reload” button on the browser. This means IE does use any items that are in the cache and instead re-requests everything on that page. In short, for IE6 and IE7, a <META> refresh will nullify any HTTP caching.

Its best to see an example. Let’s say we have a page, start.html, which contains a <META> refresh tag that redirects to main.html. The <META> Refresh tag looks like this <META http-equiv=”Refresh” content=”0;main.html”> Let’s say main.html has 3 images on it. All of those images are served with a far future Expires header. This means repeat visitors should have all 3 images referenced by main.html cached. Here is what happens:

• The visitor clicks a link to start.html.
• start.html uses a <META> refresh to send the visitor to main.html.
• Visitor’s IE browser fetches main.html.
• Visitor’s IE browser does not use the cached images. Instead it sends 3 conditional GET requests to the web server for the 3 images with If-Not-Modified headers.

There were already several reasons not to use a <META> tag to perform a refresh. Zoompf Check #99 (one of the first checks we wrote) flags on web pages that used <META> tag for redirects. Originally we flagged META refreshes because of it was a bloated and oversized solution as well all the problems <META> refreshes cause with web crawlers and accessibility. Zoompf’s remediation advice was to use an HTTP redirect and we flagged this as a low severity issue. In light of these IE performance problems, we have changed the severity to a high (which is the same severity as not using caching at all).

Want to see what performance problems your website has? META Refresh Tag Used As Redirect is just one of the 300+ web performance issues Zoompf detects when scanning your web applications. Get your instant free web performance assessment at Zoompf.com today!

Comments (1)

1 Comment »

1. 

   Hi
   Very nice and intrestingss story.

   Comment by JassiMostru — June 5, 2010 @ 7:01 am

RSS feed for comments on this post. TrackBack URL

Leave a comment

Name (required)

Mail (will not be published) (required)

Website