方法一: 使用浮动

.lineContainer {
    overflow: hidden; /* clear the float */
    border: 1px solid #000
}
.lineContainer div {
    height: 20px
} 
.left {
    width: 100px;
    float: left;
    border-right: 1px solid #000
}
.right {
    overflow: hidden;
    background: #ccc
}

<div class="lineContainer">
    <div class="left">left</div>
    <div class="right">right</div>
</div>

方法二: 使用flexbox布局

.container {
    display: flex;
}
.container > div {
    border: 1px solid black;
    height: 10px;
}

.left {
   width: 100px;
}

.right {
    width: 100%;
    background-color:#ddd;
}

<div class="container">
  <div class="left"></div>
  <div class="right"></div>
</div>

方法三:

.lineContainer {
    display:table;
    border-collapse:collapse;
    width:100%;
}
.lineContainer div {
    display:table-cell;
    border:1px solid black;
    height:10px;
}
.left {
    width:100px;
}

<div class="lineContainer">
    <div class="left">left</div>
    <div class="right">right</div>
</div>