We want to enforce all our pages to explicitly specify xml prolog in order to avoid troubles with encodings (unfortunately we are not ASCII-speaking). We want to do that by specifying invalid defaultMarkupEncoding. This way we will get exceptions for all htmls that didn't declare encoding. However it is not possible at the moment because many wicket built-in html files do not have xml prolog.
Fix is very simple: just add <?xml version="1.0" encoding="UTF-8" ?> to your html files.