.aside{
        float: left;
        width: 200px;
        background-color: red;
    }
    .content{
        margin-left: 200px;
        background-color: blue;
    }
    
<div class="aside">
    Lorem ipsum dolor sit amet, consectetur adipisicing elit. Labore provident dolor in voluptatum tempore non, placeat obcaecati totam ut delectus libero excepturi distinctio asperiores odit nobis soluta, esse aspernatur beatae.
</div>
<div class="content">
    Lorem ipsum dolor sit amet, consectetur adipisicing elit. Labore provident dolor in voluptatum tempore non, placeat obcaecati totam ut delectus libero excepturi distinctio asperiores odit nobis soluta, esse aspernatur beatae.
</div>

.aside{
    float: left;
    margin-right: -200px;
    width: 200px;
    background-color: red;
}
.content{
    float: right;
}
.content .inner{
    margin-left: 200px;
    background-color: blue;
}
<div class="aside">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Labore provident dolor in voluptatum tempore non, placeat obcaecati totam ut delectus libero excepturi distinctio asperiores odit nobis soluta, esse aspernatur beatae.
</div>
<div class="content">
<div class="inner">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Mollitia labore facere delectus quaerat expedita, tenetur recusandae tempora eos natus suscipit voluptatum necessitatibus soluta voluptates modi ullam. Ab minima similique, illum!
</div>
</div>

.aside{

float: left;
width: 200px;
}
.content{

float: right;
calc:(100% - 200px);
}
<div class="aside"> 
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Labore provident dolor in voluptatum tempore non, placeat obcaecati totam ut delectus libero excepturi distinctio asperiores odit nobis soluta, esse aspernatur beatae.
</div>
<div class="content">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Labore provident dolor in voluptatum tempore non, placeat obcaecati totam ut delectus libero excepturi distinctio asperiores odit nobis soluta, esse aspernatur beatae.
</div>

body{
display: flex;
}
.aside{

flex: 0 0 200px;
background-color: red;
}
.content{

flex: 1;
background-color:blue;
}

<div class="aside">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Fugiat magnam nihil nostrum, similique dolor deserunt voluptatem pariatur alias, ipsum, error quos quo accusantium. Deleniti neque, pariatur voluptates maxime quam perferendis!
</div>
<div class="content">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Consectetur tempora sequi minima, placeat esse nulla dolor dolores praesentium illo, distinctio fugiat eligendi debitis animi ut impedit, magni vitae officiis, architecto.
</div>