Para uma solução CSS pura, role após a 2ª <hr>. A primeira é a resposta inicial (devolvida em 2016)
A principal falha das soluções acima é que elas têm uma altura fixa para o rodapé.
E isso simplesmente não funciona no mundo real, onde as pessoas usam um zilhão de dispositivos e têm o mau hábito de girá-los quando você menos espera e ** Poof! ** lá vai o conteúdo da página por trás do rodapé!
No mundo real, você precisa de uma função que calcule a altura do rodapé e ajuste dinamicamente o conteúdo da página padding-bottompara acomodar essa altura. E você precisa executar esta pequena função na página loade nos resizeeventos, bem como no rodapé DOMSubtreeModified(apenas no caso de o rodapé ser atualizado dinamicamente de forma assíncrona ou conter elementos animados que mudam de tamanho quando interagidos).
Aqui está uma prova de conceito, usando jQuery v3.0.0e Bootstrap v4-alpha, mas não há razão para que ele não funcione nas versões inferiores de cada um.
jQuery(document).ready(function($) {
$.fn.accomodateFooter = function() {
var footerHeight = $('footer').outerHeight();
$(this).css({
'padding-bottom': footerHeight + 'px'
});
}
$('footer').on('DOMSubtreeModified', function() {
$('body').accomodateFooter();
})
$(window).on('resize', function() {
$('body').accomodateFooter();
})
$('body').accomodateFooter();
window.addMoreContentToFooter = function() {
var f = $('footer');
f.append($('<p />', {
text: "Human give me attention meow flop over sun bathe licks your face wake up wander around the house making large amounts of noise jump on top of your human's bed and fall asleep again. Throwup on your pillow sun bathe. The dog smells bad jump around on couch, meow constantly until given food, so nap all day, yet hiss at vacuum cleaner."
}))
.append($('<hr />'));
}
});
body {
min-height: 100vh;
position: relative;
}
footer {
background-color: rgba(0, 0, 0, .65);
color: white;
position: absolute;
bottom: 0;
width: 100%;
padding: 1.5rem;
display: block;
}
footer hr {
border-top: 1px solid rgba(0, 0, 0, .42);
border-bottom: 1px solid rgba(255, 255, 255, .21);
}
<link href="http://v4-alpha.getbootstrap.com/dist/css/bootstrap.min.css" rel="stylesheet">
<link href="http://v4-alpha.getbootstrap.com/examples/starter-template/starter-template.css" rel="stylesheet">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.2.0/js/tether.min.js"></script>
<script src="http://v4-alpha.getbootstrap.com/dist/js/bootstrap.min.js"></script>
<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse fixed-top">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarsExampleDefault" aria-controls="navbarsExampleDefault" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<a class="navbar-brand" href="#">Navbar</a>
<div class="collapse navbar-collapse" id="navbarsExampleDefault">
<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>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown01" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="text" placeholder="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>
</nav>
<div class="container">
<div class="starter-template">
<h1>Feed that footer - not a game (yet!)</h1>
<p class="lead">You will notice the body bottom padding is growing to accomodate the height of the footer as you feed it (so the page contents do not get covered by it).</p><button class="btn btn-warning" onclick="window.addMoreContentToFooter()">Feed that footer</button>
<hr />
<blockquote class="lead"><strong>Note:</strong> I used jQuery <code>v3.0.0</code> and Bootstrap <code>v4-alpha</code> but there is no reason why it shouldn't work with lower versions of each.</blockquote>
</div>
</div>
<footer>I am a footer with dynamic content.
<hr />
</footer>
Inicialmente, publiquei esta solução aqui, mas percebi que ela pode ajudar mais pessoas se postada nesta pergunta.
Nota: Eu tenho propositadamente embrulhou o $([selector]).accomodateFooter()como um plugin jQuery, para que pudesse ser executado em qualquer elemento DOM, como na maioria dos layouts não é o $('body')'s bottom-paddingque as necessidades de ajuste, mas alguma página invólucro elemento com position:relative(geralmente o pai imediato do footer) .
Edição posterior (mais de 3 anos após a resposta inicial):
Neste ponto, não considero mais aceitável o uso do JavaScript para posicionar um rodapé de conteúdo dinâmico na parte inferior da página. Isso pode ser alcançado apenas com CSS, usando flexbox, extremamente rápido e em vários navegadores.
Aqui está:
// Left this in so you could inject content into the footer and test it:
// (but it's no longer sizing the footer)
function addMoreContentToFooter() {
var f = $('footer');
f.append($('<p />', {
text: "Human give me attention meow flop over sun bathe licks your face wake up wander around the house making large amounts of noise jump on top of your human's bed and fall asleep again. Throwup on your pillow sun bathe. The dog smells bad jump around on couch, meow constantly until given food, so nap all day, yet hiss at vacuum cleaner."
}))
.append($('<hr />'));
}
.wrapper {
min-height: 100vh;
padding-top: 54px;
display: flex;
flex-direction: column;
}
.wrapper>* {
flex-grow: 0;
}
.wrapper>main {
flex-grow: 1;
}
footer {
background-color: rgba(0, 0, 0, .65);
color: white;
width: 100%;
padding: 1.5rem;
}
footer hr {
border-top: 1px solid rgba(0, 0, 0, .42);
border-bottom: 1px solid rgba(255, 255, 255, .21);
}
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css" integrity="sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.4.1.slim.min.js" integrity="sha384-J6qa4849blE2+poT4WnyKhv5vZF5SrPo0iEjwBvKU7imGFAV0wwj1yYfoRSJoZ+n" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.0/dist/umd/popper.min.js" integrity="sha384-Q6E9RHvbIyZFJoft+2mJbHaEWldlvI9IOYy5n3zV9zzTtmI3UksdQRVvoxMfooAo" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js" integrity="sha384-wfSDF2E50Y2D1uUdj0O3uMBJnjuUD4Ih7YwaYd1iqfktj0Uod8GCExl3Og8ifwB6" crossorigin="anonymous"></script>
<div class="wrapper">
<nav class="navbar navbar-expand-lg navbar-light bg-light fixed-top">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<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 dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>
</nav>
<main>
<div class="container">
<div class="starter-template">
<h1>Feed that footer - not a game (yet!)</h1>
<p class="lead">Footer won't ever cover the body contents, as its not fixed. It's simply placed at the bottom when the page should be shorter using `min-height:100vh` on container and using flexbox to push it down.</p><button class="btn btn-warning" onclick="addMoreContentToFooter()">Feed that footer</button>
<hr />
<blockquote class="lead">
<strong>Note:</strong> This example uses current latest versions of jQuery (<code>3.4.1.slim</code>) and Bootstrap (<code>4.4.1</code>) (unlike the one above).
</blockquote>
</div>
</div>
</main>
<footer>I am a footer with dynamic content.
<hr />
</footer>
</div>