Here is one way of doing it.
If you HTML looks like this:
<div>Contact Details
<button type="button" class="edit_button">My Button</button>
</div>
apply the following CSS:
div {
border-bottom-width: 1px;
border-bottom-style: solid;
border-bottom-color: gray;
overflow: auto;
}
.edit_button {
float: right;
margin: 0 10px 10px 0; /* for demo only */
}
The trick is to apply overflow: auto to the div, which starts a new block formatting context. The result is that the floated button is enclosed within the block area defined by the div tag.
You can then add margins to the button if needed to adjust your styling.
In the original HTML and CSS, the floated button was out of the content flow so the border of the div would be positioned with respect to the in-flow text, which does not include any floated elements.
See demo at: http://jsfiddle.net/audetwebdesign/AGavv/