STEPS TO REPRODUCE:
(1) Set your OS to use a dark theme (to trigger the dark-mode rendering for CSSWG drafts)
(2) View Example 4 in the flexbox ED, roughly here:
https://drafts.csswg.org/css-flexbox-1/#collapsed-flex-item

ACTUAL RESULTS:
The example has ~unreadable light-gray text on a white background. Screenshot:

EXPECTED RESULTS:
Readable text in the example.