L 形 div 容器

我正在尝试使用 CSS 绘制以下“L”形 div。

我怎样才能绘制这个形状，以便在这个 L 形容器中包含文本流？

Like so:

HTML:

<div class="container">
    <div class="mask"></div>
     <p>Lorem ipsum dolor sit amet, consectetur adipisici elit, sed eiusmod tempor incidunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquid ex ea commodi consequat. Quis aute iure reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint obcaecat cupiditat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
</div>

CSS:

.container
{
    border: 3px solid black;
}

.mask
{
    border-style: solid;
    border-width: 0 0 3px 3px;
    position: relative;
    float: right;
    clear: none;
    right: -3px;
    top: -3px;
    background-color: white;
    width: 50%;
    height: 4em;
}

Demo