Why TLS 1.3 isn't in browsers yet

The Transport Layer Security (TLS) protocol, which ensures secure web browsing with HTTPS, is undergoing its first major overhaul with the introduction of TLS 1.3. However, none of the major browsers have enabled TLS 1.3 by default yet due to issues with middleboxes - network appliances designed to monitor and sometimes intercept HTTPS traffic inside corporate environments and mobile networks. The original design of TLS lent itself to this type of failure as implementers implemented to the reality of the protocol, not the intention of its designer or the text of the specification. To prevent servers from ossifying in future, David Benjamin proposed a way to keep the most important joints in TLS oiled through his GREASE proposal for TLS.