HTML5 & Bootstrap Class="Container", Can It Be Applied to Body or Only Div

HTML5 & Bootstrap class=container, can it be applied to body or only div?

I recommend sticking to the

<body>
   <div class="container">
      <div class="row">
          <div class="col-md-12"> ...

format.

If you intend to work with a lot other developers or with bootstrap templates- you will see that the container classes typically nest row class divs.

Since we are talking about markup there is no right answer, but following this convention is strongly recommended.

For consistency
For easy changes to styling & reusability with other projects- this even opens the door to drop-in replacements of css stylesheets from other projects or bootstrap templates. (I have had some surprisingly good results with this).

However, if you insist on giving non-div tags "container" and "col-X" tags, be consistent. I wouldn't recommend it though and would consider any template that follows its own convention to be an indicator of poor code quality.

How to center content in a Bootstrap column?

Use:

<!-- unsupported by HTML5 -->
<div class="col-xs-1" align="center">

instead of

<div class="col-xs-1 center-block">

You can also use bootstrap 3 css:

<!-- recommended method -->
<div class="col-xs-1 text-center">

Bootstrap 4 now has flex classes that will center the content:

<div class="d-flex justify-content-center">
   <div>some content</div>
</div>

Note that by default it will be x-axis unless flex-direction is column

How to align 3 divs (left/center/right) inside another div?

With that CSS, put your divs like so (floats first):

<div id="container">
  <div id="left"></div>
  <div id="right"></div>
  <div id="center"></div>
</div>

P.S. You could also float right, then left, then center. The important thing is that the floats come before the "main" center section.

P.P.S. You often want last inside #container this snippet: <div style="clear:both;"></div> which will extend #container vertically to contain both side floats instead of taking its height only from #center and possibly allowing the sides to protrude out the bottom.

Maintain the aspect ratio of a div with CSS

Just create a wrapper <div> with a percentage value for padding-bottom, like this:

.demoWrapper {
  padding: 10px;
  background: white;
  box-sizing: border-box;
  resize: horizontal;
  border: 1px dashed;
  overflow: auto;
  max-width: 100%;
  height: calc(100vh - 16px);
}

div {
  width: 100%;
  padding-bottom: 75%;
  background: gold; /** <-- For the demo **/
}

<div class="demoWrapper">
  <div></div>
</div>

How to align a div to the middle (horizontally/width) of the page

<body>
    <div style="width:800px; margin:0 auto;">
        centered content
    </div>
</body>

HTML5 & Bootstrap Class="Container", Can It Be Applied to Body or Only Div