Generated by SassMeister.com.

SassMeister-input-HTML.haml

%header
  %h1 Flexbox - main axis alignment
%article
  %section.flex-container
    .box.flex1
      %h3 One
    .box.flex1
      %h3 Two
    .box.flex2
      %h3 Three
    .box.flex1
      %h3 Four
  %section.flex-container.column
    .box.flex1
      %h3 One
    .box.flex1
      %h3 Two
    .box.flex2
      %h3 Three
    .box.flex1
      %h3 Four
  %h1 Flexbox - cross axis alignment
  %section.flex-container.cross-axis
    .box.flex1
      %h3 One
    .box.flex1
      %h3 Two
    .box.flex2
      %h3 Three
    .box.flex1
      %h3 Four

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----

@import "compass";

// Documentation about Flex
// http://www.w3.org/TR/css3-flexbox/

$colors: (chartreuse, royalblue, plum, teal, lightsteelblue, sienna, maroon);

body {
  width: 80%;
  margin: 0 auto;
  padding: 2%;
  line-height: 1.6;
}

.flex-container {
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  margin: 1em 0;
  border: 1px solid #333;
  // flex-start, flex-end, space-between, space-around
  @include justify-content(space-between);
  
  // flex-flow is a shorthand for flex-direction & flex-wrap
  &.row {
    @include flex-flow(row);
  }
  
  &.column {
    @include flex-flow(column);
    height: 500px;
  }
  
  &.cross-axis {
    height: 200px;
    // default stretch
    // flex-start, flex-end, stretch, baseline
    @include align-items(baseline);
  }
}

.box {
  text-align: center;
}

@for $n from 1 through 4 {
  .box:nth-child(#{$n}) {
    background-color: nth($colors, $n);
  }
}

.flex1 {
  width: 100px;
  .column & {
    width: 100%;
  }
}
.flex2 {
  width: 200px;
  .column & {
    width: 100%;
  }
  .cross-axis & {
    margin-top: 20px;
    height: 100px;
  }
}

SassMeister-output.css

body {
  width: 80%;
  margin: 0 auto;
  padding: 2%;
  line-height: 1.6;
}

.flex-container {
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  margin: 1em 0;
  border: 1px solid #333;
  -webkit-justify-content: space-between;
  justify-content: space-between;
}
.flex-container.row {
  -webkit-flex-flow: row;
  flex-flow: row;
}
.flex-container.column {
  -webkit-flex-flow: column;
  flex-flow: column;
  height: 500px;
}
.flex-container.cross-axis {
  height: 200px;
  -webkit-align-items: baseline;
  align-items: baseline;
}

.box {
  text-align: center;
}

.box:nth-child(1) {
  background-color: chartreuse;
}

.box:nth-child(2) {
  background-color: royalblue;
}

.box:nth-child(3) {
  background-color: plum;
}

.box:nth-child(4) {
  background-color: teal;
}

.flex1 {
  width: 100px;
}
.column .flex1 {
  width: 100%;
}

.flex2 {
  width: 200px;
}
.column .flex2 {
  width: 100%;
}
.cross-axis .flex2 {
  margin-top: 20px;
  height: 100px;
}

SassMeister-rendered.html

<header>
  <h1>Flexbox - main axis alignment</h1>
</header>
<article>
  <section class='flex-container'>
    <div class='box flex1'>
      <h3>One</h3>
    </div>
    <div class='box flex1'>
      <h3>Two</h3>
    </div>
    <div class='box flex2'>
      <h3>Three</h3>
    </div>
    <div class='box flex1'>
      <h3>Four</h3>
    </div>
  </section>
  <section class='flex-container column'>
    <div class='box flex1'>
      <h3>One</h3>
    </div>
    <div class='box flex1'>
      <h3>Two</h3>
    </div>
    <div class='box flex2'>
      <h3>Three</h3>
    </div>
    <div class='box flex1'>
      <h3>Four</h3>
    </div>
  </section>
  <h1>Flexbox - cross axis alignment</h1>
  <section class='flex-container cross-axis'>
    <div class='box flex1'>
      <h3>One</h3>
    </div>
    <div class='box flex1'>
      <h3>Two</h3>
    </div>
    <div class='box flex2'>
      <h3>Three</h3>
    </div>
    <div class='box flex1'>
      <h3>Four</h3>
    </div>
  </section>
</article>