How can I let a div fill 100% width if no other elements are beside it?

Question

I have simple markup like this (a tab menu):

Answer 1

You mean like flexbox?

.container {
  display: flex;
  height: 50px;
  margin-bottom: 1em;
}
[class*="tab"] {
  flex: 1;
  border: 1px solid red;
}

Or CSS Tables

.container {
  display: table;
  height: 50px;
  width: 100%;
}
[class*="tab"] {
  display: table-cell;
  border: 1px solid red;
}