官方地址
https://getbootstrap.com/docs/4.4/utilities/borders/
其他文档地址
菜鸟教程
https://www.runoob.com/bootstrap4/bootstrap4-utilities.html
http://bs4.vx.link
http://bs4.vx.link/index.html?tmpui_page=/pages/utilities.html
增加
.border
.border-top
.border-right
.border-bottom
.border-left
增加
</div>
<div class="border-top" style="display: inline-block; width: 100px; height: 100px; background-color: #ccc;"></div>
<div class="border-right" style="display: inline-block; width: 100px; height: 100px; background-color: #ccc;"></div>
<div class="border-bottom" style="display: inline-block; width: 100px; height: 100px; background-color: #ccc;"></div>
&l
