I have a use case where I need to control whether or not to use a proxy using system properties, without having access to the code.

If org.htmlunit.HttpWebConnection#createHttpClientBuilder() set builder.useSystemProperties(); ("Use system properties when creating and configuring default implementations") this works.

This sets more than just proxy config according to https://issues.apache.org/jira/browse/HTTPCLIENT-1128 it sets:

ssl.TrustManagerFactory.algorithm
javax.net.ssl.trustStoreType
javax.net.ssl.trustStore
javax.net.ssl.trustStoreProvider
javax.net.ssl.trustStorePassword
java.home
ssl.KeyManagerFactory.algorithm
javax.net.ssl.keyStoreType
javax.net.ssl.keyStore
javax.net.ssl.keyStoreProvider
javax.net.ssl.keyStorePassword
http.proxyHost
http.proxyPort
http.nonProxyHosts
http.keepAlive
http.maxConnections

Personally I would like this on by default in HtmlUnit. But having a config option to set this would be useful too.