This project includes a very basic inline PostCSS plugin that rewrites a CSS class to always contain the current timestamp in the content property.
It registers dependency.txt as a dependency, and therefore the CSS should be rebuilt any time dependency.txt changes.
Gulp seems to ignore this dependency, and does not rebuild when the file changes, even though it is registered as a dependency by the plugin.
You can demonstrate this by running npm run dev and making changes to dependency.txt, and seeing the timestamp in dest/styles.css remain unchanged.
In contrast, if you run npm run build:postcss, I am building the same CSS file using postcss-cli, and changes to dependency.txt always cause the CSS to be rebuilt, which you can observe by looking at the contents of out.css.