If you're only targeting Firefox, Opera, and Chrome, not IE, and you're only targeting modern browsers (released in the ~past year), what is the best way of creating multi-level indentation of options in a select without using optgroups? That is, when you want the parent options to still be selectable, as well as the children?

<select>
    <option>
    <option>
        <option>
        <option>
        <option>
    <option>
        <option>
        <option>
            <option>
            <option>
            <option>
        <option>
        <option>
    <option>
    <option>
</select>

It would be preferable, though I don't know if it's actually possible yet, to have actual parent/child relationships between levels of options, but if it's not, at least a way of making it look like there is, where the user-experience is similar between the three browsers?