两个：first-child

div:first-child{
    color:#f00;
}
section:first-child{
    color:#0f0;
}


<div>
        <div>0-1</div>
        <div>0-2</div>
        <div>0-3</div>
    </div>
    <section>
        <div>1-1</div>
        <div>1-2</div>
        <div>1-3</div>
    </section>
    <section>
        <div>2-1</div>
        <div>2-2</div>
        <div>2-3</div>
    </section>
    
    先设置了div:first-child后
    再设置section:first-child没效果，什么原因呢