1、循序漸進

    <style>
        /* 基礎樣式（刪除） */
        
        body,
        html {
            padding: 0;
            margin: 0;
            height: 100vh;
            width: 100vw;
            display: flex;
            justify-content: center;
            align-items: center;
            flex-wrap
 
: wrap;
        }
        
        .dxd_wdh {
            width: 300px;
            height: 300px;
            background-color: rgb(43, 193, 219);
            position: relative;
        }
        /* 基礎樣式（刪除） */
        
         :root {
            --color: rgb(118, 120, 223);
        }
        
        .dxd_wdh:before,
        .dxd_wdh:after 
 
{
            content: "";
            display: block;
            width: 0;
            height: 0;
            border: 3px solid transparent;
            box-sizing: border-box;
            position: absolute;
        }
        
        .dxd_wdh:before {
            top: 0;
            left: 0;
            transition
 
: border-color 0s ease-in 0.8s, width 0.2s ease-in 0.6s, height 0.2s ease-in 0.4s;
        }
        
        .dxd_wdh:after {
            right: 0;
            bottom: 0;
            transition: border-color 0s ease-in 0.4s, width 0.2s ease-in 0.2s, height 0.2s ease-in;
        }
        
        .dxd_wdh:hover:before {
            width: 100%;
            height: 100%;
            transition: width 0.2s ease-in, height 0.2s ease-in 0.2s;
            border-top-color: var(--color);
            border-right-color: var(--color);
        }
        
        .dxd_wdh:hover:after {
            width: 100%;
            height: 100%;
            transition: border-color 0s ease-in 0.4s, width 0.2s ease-in 0.4s, height 0.3s ease-in 0.6s;
            border-bottom-color: var(--color);
            border-left-color: var(--color);
        }
    </style>

    <div class="dxd_wdh"></div>