This blog is closed as of 2/2015. @EricLaw left Microsoft in 2012, but was named an IE MVP in '13 & an IE userAgent ( in '14.

Bugs in IE8's Lookahead Downloader

Bugs in IE8's Lookahead Downloader

All bugs mentioned in this post are now fixed. 

Internet Explorer has a number of features designed to render pages more quickly. One of these features is called the "Lookahead Downloader" and it's used to quickly scan the page as it comes in, looking for the URLs of resources which will be needed later in the rendering of the page (specifically, JavaScript files). The lookahead downloader runs ahead of the main parser and is much simpler-- its sole job is to hunt for those resource urls and get requests into the network request queue as quickly as possible. These download requests are called "speculative downloads" because it is not known whether the resources will actually be needed by the time that the main parser reaches the tags containing the URLs. For instance, inline JavaScript runs during the main rendering phase, and such script could (in theory) actually remove the tags which triggered the speculative downloads in the first place. However, this "speculative miss" corner case isn't often encountered, and even if it happens, it's basically harmless, as the speculative request will result in downloading a file which is never used.

IE8 Bugs and their impact
Unfortunately, since shipping IE8, we've discovered two problems in the lookahead downloader code that cause Internet Explorer to make speculative requests for incorrect URLs. Generally this has no direct impact on the visitor's experience, because when the parser actually reaches a tag that requires a subdownload, if the speculative downloader has not already requested the proper resource, the main parser will at that time request download of the proper resource. If your page encounters one of these two problems, typically:

  • The visitor will not notice any problems like script errors, etc
  • The visitor will have a slightly slower experience when rendering the page because the speculative requests all "miss"
  • Your IIS/Apache logs will note requests for non-existent or incorrect resources

If your server is configured to respond in some unusual way (e.g. logging the user out) upon request of a non-existent URL, the impact on your user-experience may be more severe.

The BASE Bug

Update: The BASE bug is now

The first problem is that the speculative downloader "loses" the <BASE> element after its first use. This means that if your page at URL A contains a tag sequence as follows:

<html><head><base href=B><script src=relC><script src=relD><script src=relE><body>

which requests 3 JavaScript files from the path specified in "B", IE8's speculative downloader will incorrectly request download of URLs "B+relC", and "A+relD" and "A+relE". Correct behavior is to request download of URLs "B+relC", "B+relD", and "B+relE". Hence, in this case, two incorrect requests are sent, usually resulting in 404s from the server. Of course, when the main parser gets to these script tags, it will determine that "B+relC" is already available, but "B+relD", and "B+relE" have not yet been requested, and it will request those correct two URLs and complete rendering of the page.

At present, there is no simple workaround for this issue. Technically, the following syntax will result in proper behavior:

 <html><head><base href=B><script src=relC><base href=B><script src=relD><base href=B><script src=relE><body>

...but this is not standards-compliant and is not recommended. If the page removes its reliance upon the BASE tag, the problem will no longer occur.

Remember: The BASE bug is now fixed.

The Missing 4k Bug

Update: The 4k bug is now fixed. 

The second problem is significantly more obscure, although a number of web developers have noticed it and filed a bug on Connect. Basically, the problem here is that there are a number of tags which will cause the parser and lookahead downloader to restart scanning of the page from the beginning. One such tag is the META HTTP-EQUIV Content-Type tag which contains a CHARSET directive. Since the CHARSET specified in this tag defines what encoding is used for the page, the parser must restart to ensure that is parsing the bytes of the page in the encoding intended by the author. Unfortunately, IE8 has a bug where the restart of the parser may cause incorrect behavior in the Lookahead downloader, depending on certain timing and network conditions.

The incorrect behavior occurs if your page contains a JavaScript URL which spans exactly the 4096th byte of the HTTP response. If such a URL is present, under certain timing conditions the lookahead downloader will attempt to download a malformed URL consisting of the part of the URL preceding the 4096th byte combined with whatever text follows the 8192nd byte, up to the next quotation mark. Web developers encountering this problem will find that their logs contain requests for bogus URLs with long strings of URLEncoded HTML at the end.

As with the previous bug, end users will not typically notice this problem, but examination of the IIS logs will show the issue.

For many instances of this bug, a workaround is available-- the problem only appears to occur when the parser restarts, so by avoiding parser restarts, you can avoid the bug.  By declaring the CHARSET of the page using the HTTP Content-Type header rather than specifying it within the page, you can remove one cause of parser restarts.

So, rather than putting

<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">

In your HEAD tag, instead, send the following HTTP response header:

Content-Type: text/html; charset=utf-8

Note that specification of the charset in the HTTP header results in improved performance in all browsers, because the browser's parsers need not restart parsing from the beginning upon encountering the character set declaration. Furthermore, using the HTTP header helps mitigate certain XSS attack vectors.

Unfortunately, however, suspension of the parser (e.g. when it encounters an XML Namespace declaration) can also result in this problem, and it's not feasible for a web developer to avoid suspension of the parser.

But, remember: The 4k bug is now fixed. 

While these problems are significant, they are not so dire as some readers will conclude at first glance. The second bug, in particular, is quite rarely encountered due to its timing-related nature and the requirement that page have a JavaScript URL spanning a particular byte in the response. Encountering the second issue is not nearly as prevalent as some web developers believe-- for instance, we've heard claims that IE6, 7, and Firefox all have this problem, which is entirely untrue. Readers can easily determine if a page is hitting either bug by examining server logs, or watching network requests with Fiddler.

The IE team will continue our investigation into these bugs and, as with any reported issues, may choose to make available an IE8 update to resolve the issues.

Remember: All bugs mentioned in this post are now fixed. 

Apologies for the inconvenience, and thanks for reading!


  • @MrMotts: That's a pretty unappealing workaround, but if you were to do it, you'd definitely want to use a proper URL as the HREF.

  • We're seeing this very frequently on three of our sites.  I've tried all the fixes that I can find suggestions for online, but nothing works.  Any update on where MS is with this would be huge.

  • I'm getting this too. Come on MS, please give us a fix. I'm potentially missing out on real error notifications because of all the spam generated by this problem.

  • I just found an article that mentioned using this in the head of the HTML document to stop the IE8 issue:

    <meta http-equiv="X-UA-Compatible" content="IE=7" />

    I put that in for my 3 sites and it seems to be having a positive effect for now.  Is this a valid work around or should I stay clear of this?  Thanks!

  • @Bill: Generally speaking, I would expect that to *worsen* the problem rather than making it better. Using the X-UA-Compatible flag likely causes a parser restart.

  • Interesting.  I'll leave it be for now and see how often the error comes through.  I think I've only seen 1 since I put it in, but that could just be coincidence.

    Are there any other work arounds I could try?  I've tried the Content-Type fix among others.  None of them seemed to do the trick.

    Thanks Eric!


  • Thought I'd chime in as well.  We have 2 sites all using ms ajax.  Not high volume.

    We are seeing the 4k bug pretty freqeuntly.  Like everybody else, have tried the workarounds suggested.

    No joy.  

    Out of desperation I am giving the last suggestion above (<meta http-equiv="X-UA-Compatible" content="IE=7" />) a blast.

    No data to report yet but we'll see...

    Come on MS this has been going on for too long.

  • Has anyone heard when or if a fix will be provided?


  • The BASE bug was fixed in today's Cumulative IE8 Update.

    @tc, to reiterate on your question about the 4k issue: "Unfortunately I'm not able to make any statements or speculations about IE code fixes (either availability or timeframe). I can say that this is an issue that we're getting a significant amount of customer escalations about because the workarounds are unappealing."

  • Our users are seeing the 4K bug - it's causing 404 errors - that I cannot reproduce.  Please fix!  My client doesn't want to hear that I can't fix it and we're losing business.

  • @Jenn: Unless your client is actively monitoring their network traffic, it's unlikely that they are able to observe this problem.

    For instance, if they're seeing 404 Error Pages, they're not hitting this issue-- your problem is elsewhere.

  • We've been running our sites for a full day without any errors using this fix suggested by Bill Rowell.

    <meta http-equiv="X-UA-Compatible" content="IE=7" />

    Needs to be put before any linked files in the head.

    I guess that for some people this may have side-effects but it is working a treat for us.

  • Our site has always had this meta tag (first tag under the head tag):

    <meta http-equiv="X-UA-Compatible" content="IE=7" />

    The problem still continues.

  • RE: <meta http-equiv="X-UA-Compatible" content="IE=7" />

    Okay a few more days have passed and we have had a few errors but it seems to have been vastly reduced.

    So this is not a fix but somehow mitigates the problem slightly.

  • @JG: What you are seeing is probably some of your javascript references being pushed out of the problem area by the IE7 meta tag. This could be seen as a workaround but not a long term solution since the content in your pages changes anyway with future updates of your site.

    You can achieve the same result with an HTML-comment of the same length as your IE7 meta tag.

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