valid css font warnings

Can anyone tell me what they mean by these line warnings?
Thanks!

Warnings: URI : mystylesheet.css

Line : 30 font-family: You are encouraged to offer a generic family as a last alternative

Line : 44 font-family: You are encouraged to offer a generic family as a last alternative

Valid CSS information
BODY {
font-size : 10pt;
color : #000099;
font-family : Sans-Serif, Arial, Verdana, Helvetica;
}
.hiddenPic {
display : none;
}
.table10pt {
font-family : sans-serif, Arial, Verdana, Helvetica;
font-size : 10pt;
}
.table8pt {
font-family : Arial, Sans-Serif, Verdana, Helvetica;
font-size : 8pt;
}
.hovered {
text-align : center;
background-color : #0000a0;
border-style : solid;
border-width : 0;
border-color : #ffffff;
width : 80px;
height : 13px;
color : #ffffff;
font-family : arial, sans serif;
font-size : 13px;
cursor : default;
}
.normal {
text-align : center;
background-color : #c0c0c0;
border-style : solid;
border-width : 0;
width : 80px;
height : 13px;
border-color : #c0c0c0;
color : #000000;
font-family : arial, sans serif;
font-size : 13px;
cursor : default;
}

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