The width of any given button widget is controlled by what goes inside it. What this amounts to is either the primary or secondary icons, or neither, plus the text. The actual rendered width of the button itself isn't concretely specified, but instead is determined by the browser. Of course, this is a desirable feature of any widget—relying on the browser to compute dimensions. This approach scales well when there are lots of widgets in the UI to consider, and when there are lots of browser resolution configurations to consider.

There are, however, a few cases where the automatic width set forth by the browser isn't desirable. Think about several buttons in the same context, perhaps a div element. In all likelihood, these buttons will not render as having the same width, when this is in fact a desired property. Just because one button in the group has slightly more or slightly less text doesn't mean that we don't want them to share a consistent width...