FOUC on angular.io (possibly related to the dark theme)

[IgorMinar]

Docs or angular.io Bug Report

Description

When the angular.io website is reloaded with an empty cache, I experience FOUC (flash of unstyled content).

This didn't use to be the case until recently and might be related to our implementation of the dark theme #40102

The issue is present for both light and dark theme.

Minimal Reproduction

What's the affected URL?**

https://angular.io/

Reproduction Steps**

• go to angular.io
• refresh with shift+cmd+r

Screenshot

Browser info

desktop Chrome

Anything else relevant?

Lighthouse or webpagetest.org/test doesn't seem to notice this, which is odd because I can reproduce it via mobile viewport emulation in desktop Chrome.

https://webpagetest.org/result/210602_AiDcGQ_02e8728cf025db44f12402ac41627f27/

[gkalpak]

Duplicate of #42365. (Will be fixed by #42435.)

tl;dr It's the critical CSS inlining 😞

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