On most text editors in wide use, you select the attributes of the text you are about to type and they are applied when you enter the text.
For example email programs like Microsoft Outlook, Google Mail etc., you select the type of text, Bold, Italic,etc, the font color, the font, and the font size, and then start typing. When you type all those attributes are in effect.
The same is true for Microsoft Word, Adobe Indesign, etc.
I think since people are used to entering text in emails, they will be frustrated then the contact form doesn't work as expected.
One nice thing about the HTML editor is that it looks familiar to email users. It should also behave in a similar way.