<style>
        input {
            outline: none;
            height: 35px;
            line-height: 35px;
            border: 1px solid #ccc;
            color: #999;
            text-indent: 1rem;
            display: inline-block;
            transition: all .3s;
        }
        .active {
            border: 1px solid skyblue;
            color: #333;
        }
        .active2 {
            box-shadow: 0 0 3px 2px pink;;
        }
    </style>

<input type="text" value="手機(jī)">
    <script>
        window.onload = function () {
            document.querySelector('input').onfocus = function () {
                this.value = ""
                // 方法一：
                // this.style.color = "#333"
                // this.style.border = "1px solid skyblue"
                // 方法二：
                this.classList.add("active", "active2");
                // 方法三：
                // this.className = "active active2"
            }
            // trim() 去除空格
            document.querySelector('input').onblur = function () {
                if (this.value.trim() == "") {
                    this.value = "手機(jī)"
                    // 方法一：
                    // this.style.color = "#999"
                    // this.style.border = "1px solid #ccc"
                    // 方法二：
                    this.classList.remove("active", "active2");
                    // 方法三：
                    // this.className = ""
                }
            }
        }
    </script>