<select id=a1>
<option>123</option>
<option>456</option>
<option>789</option>
</select>
<input id=a2 value=2><input type=button value=change onclick="a1.selectedIndex=parseInt(a2.value)-1">
<option>123</option>
<option>456</option>
<option>789</option>
</select>
<input id=a2 value=2><input type=button value=change onclick="a1.selectedIndex=parseInt(a2.value)-1">
<option value=123>123</option>
<option value=456>456</option>
<option value=789>789</option>
</select>
<input id=a2 value=456 onchange="a1.value=a2.value" onkeydown="if(event.keyCode==13){this.blur()}" onmouseover=y.style.display=''>
<option value=123>123</option>
<option value=456>456</option>
<option value=789>789</option>
</select>
<input id=a2 value=45 onchange="a1.value=a2.value" onkeydown="if(event.keyCode==13){this.onchange()}" onmouseover=y.style.display=''>
<option value=123>123</option>
<option value=456>456</option>
<option value=789>789</option>
</select>
<input id=a2 value=45 onpropertychange="a1.value=a2.value">