Content-Type: "text/html"

The Context.HTML(format, ...args) is the method which sends HTML responses to the client. It accepts the value (like fmt package works).

func handler(ctx iris.Context) {
    response := "Hello, <strong>%s!</strong>"
    ctx.HTML(response, "World")
}

Result

Hello, <strong>World!</strong>

If you want to learn how to render template files or embedded views please head over to the View chapter instead.