Generated by SassMeister.com.

SassMeister-input-HTML.html

<div id="container1">
  Container 1
</div>
<div id="container2">
  Container 2
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v)
// Breakpoint (v)
// Susy (v)
// ----

@import 'compass/reset';
@import 'susy';
@import 'breakpoint';
$susy: layout(960px 4 1/4 inside-static);

$computer: 900px;
$computer-columns: 12;

body {
  @include container;
  @include susy-breakpoint(800px, 8) {
    @include container(8);
    background: black;
  }
  @include susy-breakpoint($computer, $computer-columns) {
    @include container($computer-columns);
    background: grey;
  }
  
  #container1 {
    background: blue;
    @include span(4);
    @include susy-breakpoint($computer, $computer-columns) {
      @include span(3);
    }
  }
  #container2 {
    background: red;
    @include span(4);
    @include susy-breakpoint($computer, $computer-columns) {
      @include span(9);
    }
  }
}

SassMeister-output.css

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed,
figure, figcaption, footer, header, hgroup,
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
  margin: 0;
  padding: 0;
  border: 0;
  font: inherit;
  font-size: 100%;
  vertical-align: baseline;
}

html {
  line-height: 1;
}

ol, ul {
  list-style: none;
}

table {
  border-collapse: collapse;
  border-spacing: 0;
}

caption, th, td {
  text-align: left;
  font-weight: normal;
  vertical-align: middle;
}

q, blockquote {
  quotes: none;
}
q:before, q:after, blockquote:before, blockquote:after {
  content: "";
  content: none;
}

a img {
  border: none;
}

article, aside, details, figcaption, figure, footer, header, hgroup, main, menu, nav, section, summary {
  display: block;
}

body {
  max-width: 960px;
  margin-left: auto;
  margin-right: auto;
}
body::after {
  content: " ";
  display: block;
  clear: both;
}
@media (min-width: 800px) {
  body {
    max-width: 960px;
    margin-left: auto;
    margin-right: auto;
    background: black;
  }
  body::after {
    content: " ";
    display: block;
    clear: both;
  }
}
@media (min-width: 900px) {
  body {
    max-width: 960px;
    margin-left: auto;
    margin-right: auto;
    background: grey;
  }
  body::after {
    content: " ";
    display: block;
    clear: both;
  }
}
body #container1 {
  background: blue;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  width: 100%;
  float: left;
  padding-left: 2.5%;
  padding-right: 2.5%;
}
@media (min-width: 900px) {
  body #container1 {
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
    width: 25%;
    float: left;
    padding-left: 0.8333333333%;
    padding-right: 0.8333333333%;
  }
}
body #container2 {
  background: red;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  width: 100%;
  float: left;
  padding-left: 2.5%;
  padding-right: 2.5%;
}
@media (min-width: 900px) {
  body #container2 {
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
    width: 75%;
    float: left;
    padding-left: 0.8333333333%;
    padding-right: 0.8333333333%;
  }
}

SassMeister-rendered.html

<div id="container1">
  Container 1
</div>
<div id="container2">
  Container 2
</div>