固定topbar · 前端开发

``` <!DOCTYPE html> <html lang="zh"> <head> <title>Demo</title> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1"> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script> <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script> </head> <body> <section id="demo"> <nav class="navbar navbar-expand-md navbar-dark fixed-top bg-dark"> <a class="navbar-brand" href="#">Carousel</a> <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation"> <span class="navbar-toggler-icon"></span> </button> <div class="collapse navbar-collapse" id="navbarCollapse"> <ul class="navbar-nav mr-auto"> <li class="nav-item active"> <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a> </li> <li class="nav-item"> <a class="nav-link" href="#">Link</a> </li> <li class="nav-item"> <a class="nav-link disabled" href="#">Disabled</a> </li> </ul> <form class="form-inline mt-2 mt-md-0"> <input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search"> <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button> </form> </div> </nav> </section> <h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1><h1>123</h1> </body> </html> ```