postcss-custom-properties injections for media queries

input.css

@media screen and (min-width: 320px) {
  :root {
    --size: 1em;
  }

  @media (min-height: 320px) {
    :root {
      --size: 0.5em;
    }
  }
}

:root {
  --size: 2em;
}

body {
  font-size: var(--size);
}

@media screen and (min-width: 320px) {
  p {
    font-size: var(--size);
  }
}

output.css

body {
  font-size: 2em;
}

@media screen and (min-width: 320px) {body {font-size: 1em;}}

@media screen and (min-width: 320px) {@media (min-height: 320px) {body {font-size: 0.5em;}}}

@media screen and (min-width: 320px) {
  p {
    font-size: 2em;
  }

@media screen and (min-width: 320px) {p {font-size: 1em;}}

@media screen and (min-width: 320px) {@media (min-height: 320px) {p {font-size: 0.5em;}}}
}

Note: nested media queries are valid according to specs & are subject to another plugin to transform it too ;)