<div class="parent">
    <div class="left">
        <p>left menu</p>
    </div>
    <div class="right">
        <li>right item1</li>
        <li>right item2</li>
        <li>right item3</li>
    </div>
</div>

html, body, p, ul, li {
    margin: 0;
    padding: 0;
}
div.left {
    background: #d2e3e3;
}
div.right {
    background: #77DBDB;
}

.left {
    float: left;
    width: 100px;
}
.right {
    margin-left: 100px; // 或 overflow: hidden
}

.parent {
    display: table;
    width: 100%;
    table-layout: fixed; // https://blog.csdn.net/qq_36699230/article/details/80658742
    .left, .right {
        display: table-cell;
    }
    .left {
        width: 100px;
    }
}

.parent {
    display: flex;
    .left {
        width: 100px; // 或 flex: 0 0 100px;
    }
    .right {
        flex: 1;
    }
}

.parent {
    display: table;
    width: 100%;
    // 设置table-layout: fixed; 会使单元格等宽, 因此此处不设置
    .left, .right {
        display: table-cell;
    }
    .left {
        width: 0.1%; // 宽度设置一个极小值, 由于没有设置table-layout: fixed; 所以宽度由内容决定
        white-space:nowrap;
    }
}

<div class="parent">
    <div class="column">
        <p>1</p>
    </div>
    <div class="column">
        <p>2</p>
    </div>
    <div class="column">
        <p>3</p>
    </div>
    <div class="column">
        <p>4</p>
    </div>
</div>

html, body, div, p {
    margin: 0;
    padding: 0;
}
.parent {
    width: 800px;
    border: 1px solid coral;
    .column {
        height: 30px;
        background: bisque;
        p {
            background: #f0b979;
            height: 30px;
        }
    }
}

.parent {
    margin-left: -20px;
    overflow: hidden;
    .column {
        float: left;
        width: 25%;
        padding-left: 20px;
        box-sizing: border-box;
    }
}

.parent {
    display: flex;
    .column {
        flex: 1;
        &+.column {
            margin-left: 10px;
        }
    }
}

.parent {
    display: flex;
}
.left, .right {
    flex: 1;
}