@media only screen and (max-width: 599px) {
    div.a {font-size: 15px;}
}

@media only screen and (min-width: 600px) {
    div.a {font-size: 24px;}
}

@media only screen and (min-width: 980px) {
    div.a {font-size: 28px;}
}

@media only screen and (min-width: 1260px) {
    div.a {font-size: 36px;}
}