title: macOS does'nt change dppx with scaling, bug or feature?
body: <p>I've made some tests on macOS 10.13 with dppx.</p>
  
  <p>macOS has 4 scales:</p>
  
  <ol>
  <li>1680 х 1050</li>
  <li>1440 х 900 (as default begin 2016+)</li>
  <li>1280 x 800 (as default untill 2016)</li>
  <li>1024 х 640</li>
  </ol>
  
  <p>I've cheched dppx (using <a href="https://bjango.com/articles/min-device-pixel-ratio/" rel="nofollow noreferrer">this online service</a>) and as a results, I've <strong>always got 2dppx</strong>!</p>
  
  <p>Why? What am I doing wrong? This is a bug?</p>
  
  <p>macbook resolution is 2560*1600, assuming that I have 2 dppx, real scaling is: <code>2560 / 2dppx x 1600 / 2 dppx === 1280 x 800</code></p>
  
  <p>Other scalling is:</p>
  
  <ol>
  <li>1680 х 1050 = 1.52 dppx</li>
  <li>1440 х 900 = 1.77 dppx</li>
  <li>1280 x 800 = 2 dppx</li>
  <li>1024 х 640 = 2.5 dppx</li>
  </ol>
  
  <p>I'm right? If so, why do the tools for checking dppx always write 2dppx?</p>