The ClientModeSpiceDefault determines what is the default kind of console opened if the user does not change it. Inside the code SpiceConsoleModel.getDefaultConsoleMode() the following values are expected: Native, Plugin, Auto, Html5 But in engine-config.properties the valid values are only Auto,Plugin,Native This means that if the "Html5" is set to vdc_options table directly (using psql or something) it should prefer the Html5 client - but it is not possible to set it using engine-config tool.
Verified on rhevm-3.6.5-0.1. Set ClientModeSpiceDefault by engine-config to values [Native, Plugin, Auto, Html5] For Html5, verified a running default is indeed set to Html5, on webadmin.