如何让两个父div彼此相邻

Question

我正在开发一个css / html网站，我遇到了这个问题：在我的style.css中我有两个div，一个用于侧边栏，一个用于内容。我希望侧边栏在侧面，但是当两者都存在时，内容总是低于侧边栏，除非我把它放在适当的位置：绝对。在这种情况下，当使用小屏幕时我会遇到问题，因为它们会使用另一个屏幕。

我粘贴代码，希望你能帮忙！

/* This is for content */
article, aside, figure, footer, header, hgroup, menu, nav, section {
    display: block;
}

#modal {    
    position: relative;
    width: 100%;
    display: inline-block; 
    display: table; 
    display: table-cell;
    float: none;
    border: 1px solid #CCC;
    box-shadow: 0 1px 5px #CCC;
    border-radius: 5px;
    font-family: verdana;
    margin: 25px auto;
    text align: center;
    background-color: #f2f2f2;
    overflow: auto;
}

#modal header {
    background: #f1f1f1;
    background-image: -webkit-linear-gradient( top, #f1f1f1, #CCC );
    background-image: -ms-linear-gradient( top, #f1f1f1, #CCC );
    background-image: -moz-linear-gradient( top, #f1f1f1, #CCC );
    background-image: -o-linear-gradient( top, #f1f1f1, #CCC );
    box-shadow: 0 1px 2px #888;
    padding: 10px;
}

#modal h1 {
    padding: 0;
    margin: 0;
    font-size: 16px;
    font-weight: normal;
    text-shadow: 0 1px 2px white;
    color: #323232;
    text-align: center;
}

#modal section {
    padding-right: 10px;
    padding-left: 10px;
    padding-bottom: 10px; 
    font-size: 12px;
    line-height: 175%;
    color: #333;
    text-align: center;
}

虽然这是侧边栏

/*sidebar */
#sidebar{
    position: static;
    margin-top: 15px;
    max-width: 150px;
}

#sidebar ul,
#sidebar ul li,
#sidebar ul ul {
    list-style: none;
    margin: 0;
    padding: 0;
    border: 0;
    top: 5%;
}

#sidebar ul {
    position: static;
    top: 90px;
    z-index: 1000;
}

#sidebar ul li {
    min-height: 1px;
    line-height: 1em;
    vertical-align: middle;
}

#sidebar ul li.hover,
#sidebar ul li:hover {
    position: relative;
    z-index: 599;
    cursor: default;
}

#sidebar ul ul {
    margin-top: 1px;
    visibility: hidden;
    position: absolute;
    top: 1px;
    left: 99%;
    z-index: 598;
    width: 100%;
}

#sidebar ul ul li {
    float: none;
}

#sidebar ul ul ul {
    top: 1px;
    left: 99%;
}

#sidebar ul li:hover > ul {
    visibility: visible;
}

#sidebar ul li {
    float: none;
}

#sidebar ul ul li {
    font-weight: normal;
}

/* Custom CSS Styles */
#sidebar {
    font-family: 'Lato', sans-serif;
    font-size: 18px;
    width: 180px;
}

#sidebar ul a,
#sidebar ul a:link,
#sidebar ul a:visited {
    display: block;
    color: #000000;
    text-decoration: none;
    font-weight: 300;
}

#sidebar > ul {
    float: none;
}

#sidebar ul {
    background: #a0a0a0;
}

#sidebar > ul > li {
    border-left: 3px solid #000000;
}

#sidebar > ul > li > a {
    padding: 10px 20px;
}

#sidebar > ul > li:hover {
    border-left: 3px solid #9b1a1c;
}

#sidebar ul li:hover > a {
    color: #9b1a1c;
}

#sidebar > ul > li:hover {
    background: #f6f6f6;
}

/* Sub Menu */
#sidebar ul ul a:link,
#sidebar ul ul a:visited {
    font-weight: 400;
    font-size: 14px;
}

#sidebar ul ul {
    width: 180px;
    background: none;
    border-left: 20px solid transparent;
}

#sidebar ul ul a {
    padding: 8px 0;
    border-bottom: 1px solid #eeeeee;
}

#sidebar ul ul li {
    padding: 0 20px;
    background: #fff;
}

#sidebar ul ul li:last-child {
    border-bottom: 3px solid #000000;
    padding-bottom: 10px;
}

#sidebar ul ul li:first-child {
    padding-top: 10px;
}

#sidebar ul ul li:last-child > a {
    border-bottom: none;
}

#sidebar ul ul li:first-child:after {
    content: '';
    display: block;
    width: 0;
    height: 0;
    position: absolute;
    left: -20px;
    top: 13px;
    border-left: 10px solid transparent;
    border-right: 10px solid #fff;
    border-bottom: 10px solid transparent;
    border-top: 10px solid transparent;
}