If you're looking for a JS templating solution then http://handlebarsjs.com/ is a fine solution. There's no reason why this should not work with kostache.
I'm not sure I understand your second paragraph there. It's a JS templating solution, so it's written in JS. If the user has JS disabled in their browser then the templates will not work, but then neither would the JS that's generating them in the first place.
Edited: It sounds to me like you may be slightly confused here too. Kostache is a server side templating engine that saves you embedding PHP logic in your views. handlerbars.js is a client-side templating engine, saving you from doing DOM manipulation to update content.