[jhuckestein]

Can somebody explain what technical reason prevent me from using any scripting language to write my web-app's client-side code?

The way I see it, web-apps today are downloading javascript code that calls back to the server from time to time and outputs a DOM tree. Any scripting language can do that, right? Why can't I write my client-side code in Ruby, Python or PHP?

[rbanffy]

Because you would have to add runtimes for all those languages with ties to the DOM. It's hard enough to keep JavaScript secure. It would be a nightmare to keep different implementations of JavaScript, Ruby, Python and client-side PHP secure on different operating systems.