display: inline-block

Inline elements like don't have a width property, whereas block elements do, but they appear on a new line. The inline-block setting creates a hybrid element, giving the span a width property. For example, when creating tab elements, this makes it so every tab has the same width.

Example code:

.tabs a span {
    display: inline-block;
    border-radius: 5px 5px 0 0;
    width: 100px;
    margin-right: 10px;
    text-align: center;
    background: #ccc;
    padding: 5px;