Development mode is suposed to “temporarily suspend Cloudflare’s edge caching and minification features”. It also should “allows requests for your website to temporarily bypass our cache so you can verify changes made to cached content.”, and “When Development Mode is on, all requests will be passed to the origin server.”
All quotes from CF help. They mainly say that the Dev mode is about bypassing cache, but the last one makes me wonder, if Dev mode also means that Cloudflare SSL certificate is also bypassed and origin certificate is served instead. Or not?