If your div is styled to have at least the width that is required to be able to keep the two buttons next to each other, then it will maintain that width regardless of what the browser is being resized to:
<div style="width: 200px;">
<span ><button>10000000000</button></span>
<span ><button>10000000000</button></span>
</div>
If you want to be able to tell the div to "go ahead and assume 100% width like you normally would, but always allocate at least 200px, even if this is more than 100% width", you can use min-width
:
<div style="min-width: 200px;">
<span ><button>10000000000</button></span>
<span ><button>10000000000</button></span>
</div>
Now, this doesn't work in IE6, but here's another solution for the same that does:
<div>
<div style="width: 200px;">
<!-- parent can never be smaller than its biggest child -->
</div>
<span ><button>10000000000</button></span>
<span ><button>10000000000</button></span>
</div>
Another solution, specifically to tell something not to wrap, would be to set white-space
which is the CSS equivalent of the now deprecated <nobr/>
<div style="white-space: nowrap;">
<span ><button>10000000000</button></span>
<span ><button>10000000000</button></span>
</div>
Note that this latter solution will not allow for any line breaks at all except for those manually created by a <br/>
or other block element.