It looks like you are missing something. All modern Web browser support Unicode, so the editor you need could be just the
<textarea>
element. Moreover, as it supports Unicode, it also supports all other UTFs. For example, if you have some application supporting just UTF-32 (imagine such thing for a minute), and some arbitrary Unicode text on the Web page shown by the a Web browser, which can be some text in control or anywhere else, you can copy this text from the browser to a clipboard and later past in that application — UTF-8 to UTF-32 "conversion" is done! This is just a usual property of the clipboard. The same happens in nearly all modern systems.
[EDIT]
And of course .NET supports the Unicode and all major UTFs.
[END EDIT]
Maybe you just need to learn how Unicode works. Please see, to start with:
http://en.wikipedia.org/wiki/Unicode[
^],
http://unicode.org/[
^],
http://en.wikipedia.org/wiki/BOM[
^],
http://unicode.org/faq/utf_bom.html[
^].
—SA