If CSS is render-blocking, why do we see FOUC?

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

Answer

This should help.

  1. DOM is constructed
  2. If we are still waiting for the CSSOM to be constructed, then we see FOUC
  3. CSSOM is constructed
  4. DOM and CSSOM are coalesced into the Render Tree which renders the DOM with CSS (styled content)

So the browser shows FOUC when waiting for CSS. Once the CSS is loaded, the DOM and CSSOM are merged into one tree, called the Render Tree and this is styled content.

The very fact that HTML is rendered unstyled is clear evidence that the HTML can render in the browser separately from the render tree, thus causing FOUC.

According to the Google article, the NY times site shows FOUC until the CSSOM is constructed and then the render tree is rendered. This demonstrates that rendering the render tree is different from rendering the DOM tree. The DOM tree is rendered, but unloaded CSS blocks the render tree from being rendered (note the difference). This is why the FOUC shows before the CSS is unblocked and the render tree shows.

In my opinion, this is the most comprehensive talk about this subject out there, from David Baron, Chief Engineer at Mozilla: https://vimeo.com/103108124

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