这很简单 你可以来点硬的

.clearfix:after { content: " "; visibility: hidden; display: block; height: 0; clear: both; } #first, #second{ box-sizing: border-box; -moz-box-sizing: border-box; -webkit-box-sizing: border-box; } #wrapper { width: 500px; border: 1px solid black; } #first { border: 1px solid red; float:left; width:50%; } #second { border: 1px solid green; float:left; width:50%; } <div id="first">Stack Overflow is for professional and enthusiast programmers, people who write code because they love it.</div> <div id="second">When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.</div> </div>

or the easy way #wrapper { display: flex; border: 1px solid black; } #first { border: 1px solid red; } #second { border: 1px solid green; } <div id="wrapper"> <div id="first">Stack Overflow is for professional and enthusiast programmers, people who write code because they love it.</div> <div id="second">When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.</div> </div>

还有很多其他的方法。 但我要用简单的方法。 我还想告诉你，这里有很多答案是不正确的 但我所展示的两种方法至少在HTML 5中都有效。

这很简单 你可以来点硬的

.clearfix:after { content: " "; visibility: hidden; display: block; height: 0; clear: both; } #first, #second{ box-sizing: border-box; -moz-box-sizing: border-box; -webkit-box-sizing: border-box; } #wrapper { width: 500px; border: 1px solid black; } #first { border: 1px solid red; float:left; width:50%; } #second { border: 1px solid green; float:left; width:50%; } <div id="first">Stack Overflow is for professional and enthusiast programmers, people who write code because they love it.</div> <div id="second">When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.</div> </div>

or the easy way #wrapper { display: flex; border: 1px solid black; } #first { border: 1px solid red; } #second { border: 1px solid green; } <div id="wrapper"> <div id="first">Stack Overflow is for professional and enthusiast programmers, people who write code because they love it.</div> <div id="second">When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.</div> </div>

还有很多其他的方法。 但我要用简单的方法。 我还想告诉你，这里有很多答案是不正确的 但我所展示的两种方法至少在HTML 5中都有效。

你可以使用CSS float属性将元素放在一起:

#first {
float: left;
}
#second {
float: left;
}

你需要确保包装div允许浮动的宽度，和空白等设置正确。

下面是解决方案:

#wrapper {
    width: 500px;
    border: 1px solid black;
    overflow: auto; /* so the size of the wrapper is alway the size of the longest content */
}
#first {
    float: left;
    width: 300px;
    border: 1px solid red;
}
#second {
    border: 1px solid green;
    margin: 0 0 0 302px; /* considering the border you need to use a margin so the content does not float under the first div*/
}

你的演示版本更新了;

http://jsfiddle.net/dqC8t/1/

这是正确的CSS3答案。希望这能对你有所帮助 我真的很推荐你去看这本书:https://www.amazon.com/Book-CSS3-Developers-Future-Design/dp/1593272863其实我现在已经通过看这本书做出了这个解决方案。: D

#wrapper{ display: flex; flex-direction: row; border: 1px solid black; } #first{ width: 300px; border: 1px solid red; } #second{ border: 1px solid green; } <div id="wrapper"> <div id="first">Stack Overflow is for professional and enthusiast programmers, people who write code because they love it.</div> <div id="second">When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.</div> </div>

我的方法:

<div class="left">Left</div>
<div class="right">Right</div>

CSS:

.left {
    float: left;
    width: calc(100% - 200px);
    background: green;
}

.right {
    float: right;
    width: 200px;
    background: yellow;
}