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"the ::first-line and ::first-letter pseudo-elements do not apply to flex containers, and flex containers do not contribute a first formatted line or first letter to their ancestors."

Also a mail from CSS WG mailing list confirming the expected behavior:
<a href="http://lists.w3.org/Archives/Public/www-style/2014Dec/0305.html">http://lists.w3.org/Archives/Public/www-style/2014Dec/0305.html</a></pre>
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