In order to construct the render-tree, the browser requires both DOM and CSSOM. CSSOM can only be constructed, once the CSS is downloaded. In essence, once the CSS is downloaded the page should be rendered alright. But, why do we see Flash Of Unstyled Content(FOUC) on the page? In what time window does the browser show unstyled content?

Please help me understand this.

Ref:
<https://developers.google.com/web/fundamentals/performance/critical-rendering-path/render-blocking-css>