Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

.header {
  // mobile modifier
  &--mobile {
    background: red;
  }

  // nav block
  &-nav {
    background: green;
    a { color: black };
  }
}

SassMeister-output.css

.header--mobile {
  background: red;
}

.header-nav {
  background: green;
}

.header-nav a {
  color: black;
}