伪类 before

效果

<span class="bjh-h3">变量声明和初始化</span>

.bjh-h3 {
    font-size: 18px;
    font-weight: 700;
    color: #000;
    position: relative;
    padding-left: 9px;
}
.bjh-h3:before {
    content: "";
    display: inline-block;
    height: 18px;
    width: 5px;
    background-color: #3c76ff;
    position: absolute;
    top: 0;
    left: 0;
}