第一种
border-bottom: 2px solid;
border-image: linear-gradient(90deg, rgba(255,255,255,0.00) 10%,#ffffff 30%,#ffffff 70%, rgba(255,255,255,0.00) 90%) 2 2 2 2;
![1.jpg](https://upload-images.jianshu.io/upload_images/14926774-ace3ab30883f4608.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
第二种
border: 2px solid;
border-image: linear-gradient(red,yellow) 30 30;
![2.jpg](https://upload-images.jianshu.io/upload_images/14926774-5fa99739c6753a7f.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)