Accelerated mobile pages link to css file

External stylesheets are not permitted. Use inline styling to avoid the additional request for css.

More info can be found at: https://github.com/ampproject/amphtml/blob/master/spec/amp-html-format.md#stylesheets

Authors may add custom styles to a document using a single <style amp-custom> tag in the head of the document.

i am using php pages so I do the following to add my custom css page, so i can seperate it and also include in all pages so changing only once etc.

<style amp-custom>
  <?php readfile( getcwd() . "/css/main.min.css"); ?>
</style>