Command and Conquer.
Command and Conquer playable in your web browser? Sure, no probs, here you go.
Yes, yes, it’s just a single mission demo but IT IS WRITTEN IN HTML, CSS AND JAVASCRIPT. Do you not understand? This is AMAZING.
Via The Verge.
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PS. Did you know that C&C: Tiberian Dawn and C&C: Red Alert were released as freeware by EA a while back? Lovely stuff.