##size No need to drop down, just like Taobao sellers select product category list...
optgroup This is not an attribute, it is just a node with multi-layer options. There are many labels used to divide nodes. For example, hgroup is used to organize titles, and
optgroup is used to organize data classification. Of course, this is only for performance, the actual classification is still in the database.
Three three groups, this is the optgroup and label.
multiple After the multi-select is true, press ctrl to select multiple lines