Rich Text Format is one of the most commonly supported document formats, so it should be no problem to open in it some proper text editor or word processor. With these you can either save them to HTML format, if possible, or copy the text and manually create a HTML file from it using a text editor, provided you know the HTML tags. Using a dedicated HTML editor should help with this tremendously.
Open RTF file Open HTML fileA text format conversion utility
A text editor and word processor
Note organiser software
Program for editing mathematic and scientific texts
A word processing program and text editor
A personal information manager
One of the many available document converters