Generated by SassMeister.com.

SassMeister-input-HTML.html

<p>あああああ</p>
  
<p class="item">aaaaa</p>  
  
 <ul class="box"><li>1</li>  
    <li>2</li>
   <li>3</li>
 </ul> 
  
  
  
  
  
  

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

@mixin fz($size) {
  font-size: $size + px;
  font-size: ($size / 10) * 1rem;
}

html {
  font-size:62,5%;
}

.item {
  @include fz(18);
}
.box {
  @include fz(26);
  list-style: none;
}

SassMeister-output.css

html {
  font-size: 62,5%;
}

.item {
  font-size: 18px;
  font-size: 1.8rem;
}

.box {
  font-size: 26px;
  font-size: 2.6rem;
  list-style: none;
}

SassMeister-rendered.html

<p>あああああ</p>
  
<p class="item">aaaaa</p>  
  
 <ul class="box"><li>1</li>  
    <li>2</li>
   <li>3</li>
 </ul>