Floating elements without defined widths

HTML

No defined width:
<ul>
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
</ul>

Defined width:
<ul class="b">
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
    <li>One Two Three</li>
</ul>

CSS

ul {
    overflow: hidden;
    width: 250px;
    border: 1px solid red;
}

li {
    float: left;
    background: orange;
    border: 1px solid green;
}

.b li {
    width: 80px;
}