HTML/optgroup

html
select>
	optgroup label="常緑樹">
		option>カシ/option>
		option>マツ/option>
		option>ツバキ/option>
	/optgroup>
	optgroup label="落葉樹">
		option>イチョウ/option>
		option>サクラ/option>
		option>コナラ/option>
	/optgroup>
/select>