but - if array is collection of observables?

@vamp fair enough. I wouldn't want to add too much overhead to this method, so I'm trying to think about the ideal way to test for this.

One could use:

        if(Object.prototype.toString.call(value) === '[object Array]' && value.length > 0){
            if(ko.isObservable(value[0])){
                value = ko.toJS(value);
            }
            value = value.join(' '); // .join() will call .toString() on entities
        }

Are there any other cases we should consider?

Maybe you want to solve your situation using css binding customization (in your own project without pull request)?

ko.bindingHandlers.css.update = (function(underlying){
    return function(element, valueAccessor){
        return underlying(element, function(){
            var value = ko.utils.unwrapObservable(valueAccessor());
            return (value instanceof Array)
                ? ko.toJS(value).join(' ')
                : value;
        });
    };
})(ko.bindingHandlers.css.update);

@vamp yes, clearly I could make my own solution fairly easily (which I have), however, this is a particular issue which I felt should actually be the default behavior.

As it stands, passing an array of strings into this binding (which seems like a fairly reasonable thing to do since the class attribute is essentially treated as just a space-separated list of strings) will result in not only a meaningless result, but invalid HTML.

Thanks for the code snippet though - I do like how you have done it here.