(Oracles) Testing Page Display in Mozilla FireFox



Mozilla's FireFox has difficulty displaying the National Association for State Universities and Land-Grant Colleges (NASULGC) homepage. The page displays correctly in Microsoft's Internet Explorer, which suggests that the display problems are with FireFox's.

Application Description

Mozilla's FireFox is an Internet browser available for Linux, MacOS, and Windows. At the time of writing this presentation, FireFox is pre-1.0 (a preview release for testing purposes).

Microsoft's Internet Explorer is the most widely used Internet browser. According to the W3's global usage ratings (June 2004) versions of Explorer comprise 72.8% of browsers in use.

Mozilla FireFox v0.9 Microsoft Internet Explorer v6.0.2900

Test Design

In Oracle-based testing, we compare the behavior of the program under test to the behavior of a source we consider accurate (an oracle).

A browser interprets the contents of a web page, displaying the page consistently with its interpretation. There are several published standards for the languages that browsers interpret. It is easy to test that well-formed pages are accurately rendered. The more difficult challenge is deciding what the browser should do (and evaluating what the browser does) in the face of non-standard code. Over the years, Internet Explorer and Netscape developed a stunning set of rules for interpreting badly written HTML. The typical result includes all of the key visual elements and looks reasonable, though it might not be what the author intended.

When testing a new browser, it is common to run the browser side by side with one or more competitors to see whether they render the same pages differently. Note that this is a "customer expectations" test, in contrast with a specification-based test. Even if Browser-1 acts consistently with a specification that says that a badly-written command should be ignored, customers (including website developers) will think that Browser-1 is at fault if Browser-2 correctly interprets the intent behind a badly-written command and displays the intended object correctly.

In the example that follows, we have not reviewed the actual web page code and do not know whether the HTML is correct or not.

Performing the Test

  1. Open Mozilla FireFox and Microsoft Internet Explorer.
  2. In Internet Explorer, use the address bar to navigate to http://www.nasulgc.org:
  3. In FireFox, use the address bar to navigate to http://www.nasulgc.org:
  4. Compare the two pages by scrolling through the browsers simultaneously.


Just by loading the page, we notice a difference immediately. In FireFox there is a big gap on the top of the page, whereas in Internet Explorer instead of a gap we see the main graphic:

Mozilla FireFox v0.9 Microsoft Internet Explorer v6.0.2900


On the bottom of the FireFox window, we can see that it is displaying a message saying "Transferring data from www.nasulgc.org...", signaling that the page has not loaded completely. Reloading the page or waiting for an extended period of time will not stop FireFox from waiting for more information.

A clear demonstration of a problem lies further down. Here we see FireFox is unable to correctly display the layers of text and graphics. Instead of being laid out as if in a table, the information instead appears jumbled and uninterpretable.

Mozilla FireFox v0.9 Microsoft Internet Explorer v6.0.2900

Here is another example:

Mozilla FireFox v0.9 Microsoft Internet Explorer v6.0.2900


Whether or not this is the result of poor HTML coding or an error in the browser, it is something that must be fixed within the browser. The user most likely is not aware of the coding quality of the page, but will certainly be able to see the difference between how FireFox displays it and how Internet Explorer displays it.

Similar Tests/Additional Notes

More information on this bug can be found on Mozilla's Bugzilla site.

Configuration Notes

Testing Mozilla's FireFox v0.9 on:

Created 23 June 2004 for the CSTER. Modified December 2004.

All images and written material ©Copyright Sam Oswald 2004

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