I'll explain with an example
p {color: #00ff22; font-family: arial, helvetica}
According to the question you want p to accept any sans-serif font along with the ones mentioned. That becomes
p {color:#00ff22;font-family:arial,Helvetica, sans-serif}
Go to the html view in Frontpage and just put a comma and add sans-serif. They might ask you to add other font types e.g serif and the same thing needs to be done.
I hope that helps