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HTML

              
                <section>
<h1>カワウソ</h1>

<p>カワウソ(獺、川獺)は、ネコ目(食肉目)イタチ科カワウソ亜科に属する哺乳動物の総称である。カワウソ亜科にはニホンカワウソやラッコなどが属している。</p>
<p id="btn">続きを読む…</p>
<div id="box">
  <p> 泳ぎが得意であり、水中での生活に適応している。また、ラッコ以外のカワウソは陸上でも自由に行動している。南極、オーストラリア、ニュージーランドを除く、世界全域の水辺や海上で生息している。水かきをもった四肢は短く、胴体は細長い。このような体型は水の抵抗が少なく、敏捷な泳ぎを可能にしている。</p>
</div>
</section>
              
            
!

CSS

              
                $color: #1EA6BB;
section{
  width: 500px;
  margin: 0 auto;
}
#btn{
  padding: 20px;
  background: $color;
  color: #fff;
  cursor: pointer;
  &:hover{
    background: lighten($color, 3%);
  }
}
#box{
  display: none;
  overflow: hidden;
  height: 0;
}
              
            
!

JS

              
                window.onload = function(){  

var btn = document.getElementById('btn') ;
var box = document.getElementById('box') ;  
    
var copyBox = box.cloneNode(true);
box.parentNode.appendChild(copyBox);
copyBox.style.cssText = "display:block; height:auto; visibility:hidden; " ;
var copyBoxH = copyBox.offsetHeight;
box.parentNode.removeChild(copyBox);

btn.onclick = function(){
    var boxH = box.offsetHeight;
    if(boxH < 1){
        box.style.display = "block";
        slideDown(boxH);
    } else {
        slideUp(boxH);
    }
};

var slideDown_timer;
function slideDown(boxH){
    if (boxH < copyBoxH) {
        var boxH = boxH + 5;
        box.style.height = boxH + "px";
        slideDown_timer = setTimeout(function(){slideDown(boxH)},5);
    } else {
        clearTimeout(slideDown_timer);
        box.style.height = copyBoxH;
    }
}

var slideUp_timer;
function slideUp(boxH){
    if (boxH >= 1) {
        var boxH = boxH - 5;
        box.style.height = boxH + "px";
        slideUp_timer = setTimeout(function(){slideUp(boxH)}, 5);
    } else {
        clearTimeout(slideUp_timer);
        box.style.height = 0;
        box.style.display = "none";
    }       
}

}
              
            
!
999px

Console