1) Copy the attached feature file to the dirroot of the site. 2) Execute with firefox, chrome, ie, phantomjs. It covers all the cases implemented by this issue. Should end with 0 failures. Note there are failures with safari, in "verify that every set operation clears previous one", but it's in a case not used in core (multiple editions of multi-select without saving the form). /vendor/bin/behat --config path/to/your/config \ /full/path/to/the/attached/feature/file/multiple_select.feature 3) Run the whole behat suite against all browsers. There should not be any regression in existing uses of select boxes. That's all. Thanks!