1.运行效果如图所示


2.实现代码如下

<!DOCTYPEhtml><html><head><metacharset="utf-8"><metahttp-equiv="X-UA-Compatible"content="IE=edge"><title>按钮下拉菜单</title><!--最新版本的Bootstrap核心CSS文件--><linkrel="stylesheet"href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css"integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh5u"crossorigin="anonymous"><!--可选的Bootstrap主题文件（一般不用引入）--><linkrel="stylesheet"href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap-theme.min.css"integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp"crossorigin="anonymous"></head><body><divclass="btn-group"><buttonclass="btnbtn-defaultdropdown-toggle"data-toggle="dropdown"type="button">按钮下拉菜单<spanclass="caret"></span></button><ulclass="dropdown-menu"><li><ahref="##">按钮下拉菜单项</a></li><li><ahref="##">按钮下拉菜单项</a></li><li><ahref="##">按钮下拉菜单项</a></li><li><ahref="##">按钮下拉菜单项</a></li></ul></div><scriptsrc="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script><!--最新的Bootstrap核心JavaScript文件--><scriptsrc="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa"crossorigin="anonymous"></script></body></html>