What Google says about "vary user agent"

🎥 YouTube Videos 1

Is it really necessary to use the 'Vary: User-Agent' header on all your adaptive pages?

Google 🎥 YouTube May 2015 ★★★

Pour les pages web adaptatives proposant du contenu différent selon l'agent utilisateur, il est crucial d'utiliser le header HTTP 'Vary: Use...

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What Google says about « vary user agent »