Why does this program not print anything?

Because it's a tree of the HTML. Upper level is empty. For example if you need parse all url from html:

package main

import (
        "fmt"
        "log"
        "net/http"

        "golang.org/x/net/html"
)

func main() {
        r, err := http.Get("https://google.com")
        if err != nil {
                log.Panicln(err)
        }

        defer func() {
                err := r.Body.Close()
                if err != nil {
                        fmt.Println(err)
                }
        }()

        node, err := html.Parse(r.Body)
        if err != nil {
                log.Panicln(err)
        }
        fmt.Println(node.Data)

        var f func(*html.Node)
        f = func(n *html.Node) {
            if n.Type == html.ElementNode && n.Data == "a" {
                for _, a := range n.Attr {
                    if a.Key == "href" {
                        fmt.Println(a.Val)
                        break
                    }
                }
            }
            for c := n.FirstChild; c != nil; c = c.NextSibling {
                f(c)
            }
        }
        f(node)
}