<style>
	.demo_css_sprt, .demo_css_sprt div	{
		border: 1px dotted red;
		padding: 1em;
	}
	.demo_css_sprt div	{
		margin: 1em 0;
	}
	.demo_css_sprt div.ab:before	{
		content:"【Hellow World!】 ";
	}
</style>
<div class="demo_css_sprt">
	要素1
	<div class="ab">要素2</div>
	<div class="cd">要素3</div>
</div>