HtmlUnit is an open-source GUI-less browser written in 100% Java. Because HtmlUnit does not involve any native code, debugging GWT Tests in development mode can be done entirely in a Java debugger. HtmlUnit does not require firing up a new browser process; the HtmlUnit browser instances just run as new threads.
Because HtmlUnit is a GUI-less browser, layout cannot be tested on HtmlUnit. You can annotate such test methods or classes that must not be run by HtmlUnit as @DoNotRunWith(Platform.HtmlUnit). Additionally, correct tests can sometimes fail on HtmlUnit, either because the HtmlUnit support for that feature is lacking or because of HtmlUnit's issues with flakiness when running asynchronous tests. In addition to sending us bug reports, you can annotate such tests with @DoNotRunWith so that your build does not keep on breaking. There is also a temporary option for reducing the flakiness that HtmlUnit might cause with asynchronous tests (while we fix the fundamental problem). You can specify how many times GWT should attempt to run a test in case of a failure. For example, with -Xtries 3, GWT will attempt to run a test up to three times.
The HtmlUnit runstyle enables you to specify other browser emulations. By default, GWT runs HtmlUnit in the Firefox3 emulation mode. As of the 2.0 release, GWT has not been extensively tested on the other emulations that HtmlUnit supports, namely FF2, IE6, IE7, and IE8. Still, to use them, you can define the system property gwt.args, as explained before. For example, to cause tests to run both in FF3 and IE8 emulation mode, set gwt.args to: