AppHtmlConventions.cs

    public class AppHtmlConventions : HtmlConventionRegistry
    {
        public AppHtmlConventions()
        {
//...
            Editors
                .If(def => // Condition)
                .Modify((e, tag) => tag.Attr("value", e.RawValue.ToString()));
//...
        }
    }

AppRegistry.cs

    public class AppRegistry : FubuRegistry
    {
        public AppRegistry()
        {
//..
            HtmlConvention<AppHtmlConventions>();
//...
        }
    }