how to define multiple classes' hover event in css?

You should separate with a comma, like this:

.my_div:hover, .my_td:hover {
      border: 1px solid red;
}
.contact-dpd:hover .contact-content, .work-dpd:hover .work-content{
     display:block
}

Add a comma in between: .my_div:hover, .my_td:hover.