纯CSS斜角边框
background: #4a87d8;
background:linear-gradient(-45deg, transparent 15px, #4a87d8 0) bottom right,
linear-gradient(45deg, transparent 15px, #4a87d8 0) bottom left;
background-size: 20% 50%;
background-repeat: no-repeat;
background: #4a87d8;
background:linear-gradient(-45deg, transparent 15px, #4a87d8 0) bottom right,
linear-gradient(45deg, transparent 15px, #4a87d8 0) bottom left;
background-size: 20% 50%;
background-repeat: no-repeat;