Smaller font size in signature won't show in Editor

I set Tahoma 12pt as the default font type for new e-mails and defined a signature, that is also one part in Tahoma 12tp, but one part Tahoma 10pt. When I create a new e-mail, the whole signature is Tahoma 12pt. How can I get the signature displayed as it should be.

Are you maybe composing your message in plain text?

No, in HTML.

I did notice that it works better if you first set the default font, then create the signature. For some reason if you do it the other way around it may changes the fonts in the signature.