[html] How to align flexbox columns left and right?

With typical CSS I could float one of two columns left and another right with some gutter space in-between. How would I do that with flexbox?

http://jsfiddle.net/1sp9jd32/

#container {_x000D_
  width: 500px;_x000D_
  border: solid 1px #000;_x000D_
  display: -webkit-flex;_x000D_
  display: -ms-flexbox;_x000D_
  display: flex;_x000D_
}_x000D_
#a {_x000D_
  width: 20%;_x000D_
  border: solid 1px #000;_x000D_
}_x000D_
#b {_x000D_
  width: 20%;_x000D_
  border: solid 1px #000;_x000D_
  height: 200px;_x000D_
}

<div id="container">_x000D_
  <div id="a">_x000D_
    a_x000D_
  </div>_x000D_
  <div id="b">_x000D_
    b_x000D_
  </div>_x000D_
</div>

There are different ways but simplest would be to use the space-between see the example at the end

#container {    
    border: solid 1px #000;
    display: flex;    
    flex-direction: row;
    justify-content: space-between;
    padding: 10px;
    height: 50px;
}

.item {
    width: 20%;
    border: solid 1px #000;
    text-align: center;
}

see the example

I came up with 4 methods to achieve the results. Here is demo

Method 1:

#a {
    margin-right: auto;
}

Method 2:

#a {
    flex-grow: 1;
}

Method 3:

#b {
    margin-left: auto;
}

Method 4:

#container {
    justify-content: space-between;
}

Another option is to add another tag with flex: auto style in between your tags that you want to fill in the remaining space.

https://jsfiddle.net/tsey5qu4/

The HTML:

<div class="parent">
  <div class="left">Left</div>
  <div class="fill-remaining-space"></div>
  <div class="right">Right</div>
</div>

The CSS:

.fill-remaining-space {
  flex: auto;
}

This is equivalent to flex: 1 1 auto, which absorbs any extra space along the main axis.