JSFiddle - React, Tailwind, and code Playground

HTML

<style>    
    optgroup     {  
    display: inline-block;  
    }
</style>
<select>    
    <optgroup> 
        <option>123</option> 
        <option>456</option>   
        <option>789</option>  
    </optgroup>   
    <optgroup>    
        <option class="oui">I II III</option>  
        <option class="oui">IV V VI</option>   
        <option class="oui">VII VIII IX</option>   
    </optgroup>
</select>
yes

CSS

select{

  width:500px;
}
.oui{
  margin-right: 0px;
}