CSS warning shown when map container is not added to the DOM

When creating a new map instance using a container that is not yet added to the DOM we get this warning:

"This page appears to be missing CSS declarations for Mapbox GL JS, which may cause the map to display incorrectly. Please ensure your page includes mapbox-gl.css, as described in https://www.mapbox.com/mapbox-gl-js/api/."

If the container div is added to the DOM before the map is created the warning will not show. Beside this warning, it works perfectly fine to create a map for a container that is not in the DOM. We use this approach in our React setup, where the container is rendered at a later stage.

The issue seems to be with the current _detectMissingCSS() in src/ui/map.js

mapbox-gl-js version: 1.9.0

browser: Google Chrome

Steps to Trigger Behavior

const div = document.createElement('div');

// document.body.appendChild(div);

const map = new mapboxgl.Map({
  container: div
});

Check the console to see the warning.

If the container div is added to the DOM before the map is created the warning will not show.

Link to Demonstration

https://jsfiddle.net/mastermaps/nz3Ld5ar/8/

Expected Behavior

Don't show the warning when the CSS is actually present.

Actual Behavior

The warning shows even though the CSS is loaded.

Start with _detectMissingCSS() in src/ui/map.js and reproduce the warning using a container div that has not yet been added to the DOM. Compare that behavior with a DOM-attached container and verify that the warning is suppressed when the CSS is present.