t {
  padding: 0;
}

.test {
  contain: size;
  background: lightblue;
  border: 1px solid blue;
  margin: 5px;
}
</style>

<!-- To measure the size of an empty <select> without size containment -->
<select id="reference" multiple size="1"></select>

<select multiple class="test cis-both"
data-expected-client-width="55" data-expected-client-height="66"></select>
<select multiple class="test cis-width"
data-expected-client-width="55"></select>
<select multiple class="test cis-height"
data-expected-client-hei