Differenciation between HTTP and COAP

Question

Why do browsers and web servers not replace HTTP with COAP?

Given that, HTTP is mainly used for viewing web pages. COAP is a simplified version of HTTP for IoT or WSNs. Although COAP is based on UDP, it should have ACK messages to emulate TCP. Since COAP is simpler than HTTP, it will have lower latency and draw less power.

Is it expected that COAP will completely replace HTTP? Is it just a matter of time? 

Are there any features which are supported only by HTTP?

Answer 1

The industry plan is to improve HTTP by moving to HTTP/2, and HTTP/2 includes (among other features) a header compression, which should bring you similar benefits than CoAP.While most web servers and some browsers today support HTTP/2 already, AFAIK no browser nor any server support CoAP. Same goes for TLS vs. DTLS.

Are there features COAP cannot support but HTTP can?

As you said, HTTP is TCP based, while CoAP is UDP based. UDP requires that you send UDP pings every few seconds to keep the NAT/Firewall connection open, while in TCP typically it is only required every 15 min or so. So if you need to keep the connection open (e.g. for push technologies), then CoAP is less efficient than HTTP (and HTTP/2).