Bulma currently doesn't seem to properly support <select>s with the multiple attribute. That, or I can't just seem to find it in the documentation.

What kind of support should be offered here?
multiple selects?Pitching in here, but shouldn't we at least have Bulma not interfere with the selects by default (using a :not([multiple]) I'd guess), until it's supported?
@jpmurray just dont wrap ur select in <span class="select">
I've made this little bit of css, designed to work with bulma vs changing it. It doesn't but probably should make use of any defined variables.
(excuse the ng bindings)
select[multiple].multi-select {
all: initial;
font: inherit;
overflow-y: auto;
border-radius: 3px;
padding-bottom: 0.5em;
padding-left: 0.625em;
padding-top: 0.5em;
background-color: white;
border: 1px solid #dbdbdb;
color: #363636;
cursor: pointer;
display: block;
outline: none;
padding-right: 2.5em;
}
select[multiple].multi-select:active, select[multiple].multi-select:focus {
border-color: #00d1b2;
}
<div class="field">
<label class="label">Place</label>
<p class="control">
<select class="multi-select" multiple="multiple" name="place" ng-model="vm.place" ng-required="true" ng-options="place.id as (place.name + ' ' + place.shortDescription) group by place.company.name for place in vm.places|orderBy:'name'">
</select>
</p>
</div>

It's coming in 0.4.4: http://bulma.io/documentation/form/select/
Optgroup usage doesn't appear in the example. Has it been considered? @jgthms

This feature seems to be broken. Will only select one item at a time.
Yeah this isn't working for me either. Even the live example on Bulma's website doesnt work for me on Chrome or Firefox.
EDIT: I just realized, to use it you have to either click and drag over multiple options, or hold control and click on multiple options like you would while highlighting files. That's weirdly difficult to use. Not bulma's fault.
Most helpful comment
It's coming in
0.4.4: http://bulma.io/documentation/form/select/