Edit page content from an outside webpanel

Would it be possible to edit any webpage ( text only) from an outside webpanel by adding a script in the website?

I know it's possible for wordpress website, we can do it by creating a plugin. But I'm looking for a platform independent solution.

Let me know if it's possible. If you can do it please bid the corrent ammount & time ( don't post placeholders).

Also mention if it would have any limitations or won't work for certain CMS.