I would not advice you designing your own HTML editor for the following reasons:

1. there's free stuff out there. many very talented programmers (God bless the bastards) spend months or even years developing the most sweet pieces of coding you ever seen and let you use at at will. I gave up my own editor when I found htmlArea (http://www.interactivetools.com/products/htmlarea/)

2. having made the point above, you'll be wasiting your time trying to design something that will match the quality of the above, so unless you're simply trying to learn a few techniques, don't go all the way. by all means try and re-create the software in your own way, practice and learn from the code, but anyone is better off taking advantage of the free software available.

3. I forgot wat my third point was...