您只需要浮动左元素.

HTML

<div id="left">Hello yoyoyoy jrllo hello blah blah</div>
<div id="right"></div>

CSS

#left,#right{
    height:50px;
}

#left{
    background:red;
    float:left;
}

#right{
    background:green;
}

https://jsfiddle.net/qb3374ou/

右div与内容..相同的结果 https://jsfiddle.net/qb3374ou/1/

right div with content.. same result https://jsfiddle.net/qb3374ou/1/