I selected one of the fonts to try it out, but then decided I'd rather leave it with the Outlook.com default, which is not to have a specified font at all.

Unfortunately, it appears that there is no way to remove your selection of the default font. Once you have selected a font, you cannot "clear formatting" and go back to composing an email that doesn't have specific <font> tags embedded in the HTML.

Is there any way to solve this problem?

In case this wasn't clear the first time, let me try explaining it another way:

(1) Start with a new Outlook.com account and compose a new email.

(2) While you are composing your email, go to the "Edit HTML" view, and you will see that the HTML code does not contain any <font> tags specifying a particular font. Thus, your email will be displayed in whatever font the reader is using -- you are not forcing a particular font on them. (This is what I want!)

(3) Now, go into the mail settings and change your default font to Franklin Gothic or Verdana or Garamond or whichever one of the 9 options you want.

(4) Go back to the inbox and compose a new email. Go to the "Edit HTML" view and you will see that the email now contains <font> tags specifying your chosen font.

I want to undo this selection entirely and go back to the situation described in step (2), where there is NO specified font.

I am NOT looking to choose a different default font. I want to have NO DEFAULT FONT at all -- no <font> tags embedded in the HTML, i.e. the original situation before you select a default font.

This is a very basic feature and should be added immediately.