怎么设置css字体单行居中

<style>
    .app{
        width: 200px;
        height: 100px;
        border: 1px solid skyblue;
    }
</style>
<div class="app">
    
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        border: 1px solid skyblue;
    }
    .app > p{
        text-align: center;
        overflow: hidden;　
        text-overflow:ellipsis;　　
        white-space: nowrap;
    }
</style>
<div class="app">
    <p>Hello World！</p>
                    <div class="aritcle_card">
                        <a class="aritcle_card_img" href="/xiazai/code/10761">
                            <img src="https://img.php.cn/upload/webcode/000/000/010/176362920210614.jpg" alt="通达CMS蓝色中英双语企业网站1.4.2">
                        </a>
                        <div class="aritcle_card_info">
                            <a href="/xiazai/code/10761">通达CMS蓝色中英双语企业网站1.4.2</a>
                            <p>通达CMS是采用PHP+MYSQL进行开发的。支持伪静态设置，可生成google和百度地图，支持自定义url、关键字和描述，利于收录...后台简单明了，代码简洁，采用DIV+CSS 利于SEO，企业建站系统是一套专门用于中小企业网站建设的网站管理系统。</p>
                            <div class="">
                                <img src="/static/images/card_xiazai.png" alt="通达CMS蓝色中英双语企业网站1.4.2">
                                <span>0</span>
                            </div>
                        </div>
                        <a href="/xiazai/code/10761" class="aritcle_card_btn">
                            <span>查看详情</span>
                            <img src="/static/images/cardxiayige-3.png" alt="通达CMS蓝色中英双语企业网站1.4.2">
                        </a>
                    </div>
                
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        text-align: center;
        border: 1px solid skyblue;
    }
</style>
<div class="app">
    <span>Hello World！</span>
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        text-align: center;
        border: 1px solid skyblue;
    }
    .app > span{
        line-height: 100px;
    }
</style>
<div class="app">
    <span>Hello World！</span>
</div>