Go template.ExecuteTemplate include html

Convert your []byte or string to type template.HTML (documented here)

p.Body = template.HTML(s) // where s is a string or []byte

Then, in your template, just:

{{.Body}}

It will be printed without escaping.

EDIT

In order to be able to include HTML in you page's body you need to change the Page type declaration:

type Page struct {
    Title string
    Body  template.HTML
}

then assign to it.

I created a custom function for the template as follows:

func noescape(str string) template.HTML {
    return template.HTML(str)
}

var fn = template.FuncMap{
    "noescape": noescape,
}

Then on your template:

{{ noescape $x.Body }}

Take a look at the template.HTML type. It can be used to encapsulate a known safe fragment of HTML (like the output from Markdown). The "html/template" package will not escape this this type.

type Page struct {
    Title string
    Body template.HTML
}

page := &Page{
    Title: "Example",
    Body:  template.HTML(blackfriday.MarkdownCommon([]byte("foo bar")),
}

I usually write my own func Markdown(text string) html.Template method that calls blackfriday with the appropriate config and does some type conversions. Another alternative might be also to register a "html" func in the template parser, that allows you to output any value without any escaping by doing something like {{html .MySafeStr}}. The code might look like:

var tmpl = template.Must(template.New("").Funcs(template.FuncMap{
    "html": func(value interface{}) template.HTML {
        return template.HTML(fmt.Sprint(value))
    },
}).ParseFiles("file1.html", "file2.html"))