CSS3やjQueryで、要素の幅を0から100%までアニメートし、その要素とラッパーを必要な幅だけ、事前に設定した幅なしで表示する

.wrapper {
    display: inline-block;

    height: 20px;
    width: auto;
  
    padding:10px;
  
    background:#DDD;
}

.contents {
    display:inline-block;
  
    width:0%;
  
    white-space:nowrap;
    overflow:hidden;
  
    background:#c3c;
}

.wrapper:hover .contents {
    -webkit-transition: width 1s ease-in-out;
    -moz-transition: width 1s ease-in-out;
    -o-transition: width 1s ease-in-out;
    transition: width 1s ease-in-out;

    width:100%;
}

<div class="wrapper">
    <span>+</span>
    <div class="contents">These are the contents of this div</div>
</div>

.wrapper {
    background:#DDD;
    display:inline-block;
    padding: 10px;
    height: 20px;
    width:auto;
}

.label {
    display: inline-block;
    width: 1em;
}

.contents, .contents .inner {
    display:inline-block;
}

.contents {
    white-space:nowrap;
    margin-left: -1em;
    padding-left: 1em;
}

.contents .inner {
    background:#c3c;
    width:0%;
    overflow:hidden;
    -webkit-transition: width 1s ease-in-out;
    -moz-transition: width 1s ease-in-out;
    -o-transition: width 1s ease-in-out;
    transition: width 1s ease-in-out;
}



.wrapper:hover .contents .inner {
   
    width:100%;
}

<div class="wrapper">
    <span class="label">+</span><div class="contents">
        <div class="inner">
            These are the contents of this div
        </div>
    </div>
</div>