css - How to place two divs next to each other?

Question

Consider the following code:

#wrapper {
    width: 500px;
    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>

Answer 1

Float one or both inner divs.

Floating one div:

#wrapper {
    width: 500px;
    border: 1px solid black;
    overflow: hidden; /* will contain if #first is longer than #second */
}
#first {
    width: 300px;
    float:left; /* add this */
    border: 1px solid red;
}
#second {
    border: 1px solid green;
    overflow: hidden; /* if you don't want #second to wrap below #first */
}

or if you float both, you'll need to encourage the wrapper div to contain both the floated children, or it will think it's empty and not put the border around them

Floating both divs:

#wrapper {
    width: 500px;
    border: 1px solid black;
    overflow: hidden; /* add this to contain floated children */
}
#first {
    width: 300px;
    float:left; /* add this */
    border: 1px solid red;
}
#second {
    border: 1px solid green;
    float: left; /* add this */
}