an html select tag does not show dropdown box on opera

[Colourgrey]

Colourgrey last edited by

<select>
<option value="aaa">A</option>
<option value="bbb">B</option>
</select>

on opera web browser,
I clicked button that shows dropdown box,
but I couldn't see dropwdown box.
on other browsers,dropdown box was shown when I clicked the button.

is it a bug or an error due to my miss?