How to specify the bottom border of a ?

Question

I tried :

.bb {
    border-bottom:1px;
}

But no effect at all.

Answer 1

What I want to say here is like some kind of add-on on @Suciu Lucian's answer.

The weird situation I ran into is that when I apply the solution of @Suciu Lucian in my file, it works in Chrome but not in Safari (and did not try in Firefox). After I studied the guide of styling table border published by w3.org, I found something alternative:

table.myTable{
  border-spacing: 0;
}

table.myTable td{
  border-bottom:1px solid red;
}

Yes you have to style the td instead of the tr.