By using the proper Unicode character with a TrueType font that displays the Slavonic characters, you can input the Slavonic characters into a word processing program and then share them on a website. Here is an example, which is the first part of the Lord's prayer:

(Arial font)
Отче нашъ, иже еси на небесѣхъ

(Lazov font)
Отче нашъ, иже еси на небесѣхъ

This should appear like the embedded image:


There are many different fonts which display Slavonic, so I am only listing my favorites:
Lazov

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Update: None of the Unicode fonts can properly display the Slavonic characters, and therefore should not be used. Instead, the Hirmos font is recommended, available at: http://www.irmologion.ru/fonts.html