The following information is not very difficult to find.
However, it may help to gain some time (yes, people tend to search on Google first). And I am sure I will need this information, again, one day. So, this is a good reason to put it on this blog.
I have just updated an application that uses HtmlUnit.
One of the things that annoyed me, was the regular logs about runtime errors with the StrictErrorReporter.
The application was using the java.util.logging mechanism.
To disable these logs, I updated my loggers.properties.
Here is my logging configuration for HtmlUnit.
org.apache.http.level = INFO
com.gargoylesoftware.htmlunit.level = WARNING
com.gargoylesoftware.htmlunit.html.HtmlScript.level = OFF