The control() mixin ensures consistency by providing a set of styles that are shared between all these form controls. You can use it to create additional controls:
<buttonclass="bulma-control-mixin is-small">
Small
</button><buttonclass="bulma-control-mixin">
Normal
</button><buttonclass="bulma-control-mixin is-medium">
Medium
</button><buttonclass="bulma-control-mixin is-large">
Large
</button>