<div class="wrapper">
    <!-- content -->
    <div class="push"></div>
</div>
<div class="footer">footer</div>

html, body {
  margin: 0;
  padding: 0;
  height: 100%;
}
.wrapper {
  min-height: 100%;  
  margin-bottom: -50px; /* 等于footer的高度 */
}
.footer, .push {
  height: 50px;
}

<div class="content">
  <div class="content-inside">
    <!-- content -->
  </div>
</div>
<div class="footer">footer</div>

html, body {
  margin: 0;
  padding: 0;
  height: 100%;
}
.content {
  min-height: 100%;
}
.content-inside {
  padding: 20px;
  padding-bottom: 50px;
}
.footer {
  height: 50px;
  margin-top: -50px;
}

<div class="content">
  <!-- content -->
</div>
<div class="footer">footer</div>

.content {
  min-height: calc(100vh - 70px);
}
.footer {
  height: 50px;
}

<div class="content">
  <!-- content -->
</div>
<div class="footer">footer</div>

html {
  height: 100%;
}
body {
  min-height: 100%;
  display: flex;
  flex-direction: column;
}
.content {
  flex: 1;
}

<div class="content">
  <!-- content -->
</div>
<div class="footer">footer</div>

html {
  height: 100%;
}
body {
  min-height: 100%;
  display: grid;
  grid-template-rows: 1fr auto;
}
.footer {
  grid-row-start: 2;
  grid-row-end: 3;
}