.d1 {
background: linear-gradient(to left top, transparent calc(50% - 1px), red 50%, transparent calc(50% + 1px));
}
.d2 {
border-bottom: 1px solid red;
transform: rotate(-30deg);
}
.d3 {
border-bottom: 1px solid red;
transform: skew(0, -30deg);
}