Reproduction for issue 11ty/eleventy#3007

To run:

npm i
npm start

Visit http://localhost:8080. The text "This is a test." should have a blue background, but it doesn't the first time the Dev Server is run. For more detail on the issue, see the full issue description.

Implementation notes

This repo demonstrates the use of Eleventy to build a static website with a CSS stylesheet generated by PostCSS CLI. In development (the start script defined in package.json for npm start), PostCSS CLI waits for 5 seconds to simulate a build that is not complete before Eleventy Dev Server attempts to copy the output to the _site directory.

    "start": "npm run start:eleventy & npm run start:postcss",
    "start:eleventy": "eleventy --serve --incremental",
    "start:postcss": "sleep 5 && postcss styles/styles.css --dir _tmp --watch --verbose",

  eleventyConfig.addPassthroughCopy({
    "_tmp/styles.css": "assets/styles/styles.css",
  });

Note: Rather than PostCSS CLI outputting the stylesheet directly into the _site directory, which would not benefit from Eleventy Dev Server's live reloading, PostCSS CLI outputs the file to _tmp, and then Eleventy should copy it from there into the _site directory and live-reload it in the browser whenever it changes.