em in gtk, and em with sdk

gistfile1.js

const Gtk = imports.gi.Gtk;
const Gdk = imports.gi.Gdk;
const System = imports.system;

let css = "\n\
GtkLabel {\n\
    font-size: 5em;\n\
}\n";

Gtk.init(null, 0);
let provider = new Gtk.CssProvider();
provider.load_from_data(css);
Gtk.StyleContext.add_provider_for_screen(
    Gdk.Screen.get_default(), provider,
    Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);

let label = new Gtk.Label({ label: "Should be 5em, or 60 pixels" });

let test_window = new Gtk.Window();
test_window.add(label);
test_window.connect("destroy", Gtk.main_quit);
test_window.show_all();
Gtk.main();

gistfile2.js

const Lang = imports.lang;
const Endless = imports.gi.Endless;
const Gdk = imports.gi.Gdk;
const Gtk = imports.gi.Gtk;
const GObject = imports.gi.GObject;

const TEST_APPLICATION_ID = 'com.endlessm.example.test';

const CSS = "\n\
GtkLabel {\n\
    font-size: 5em;\n\
}\n";

const TestApplication = new Lang.Class ({
    Name: 'TestApplication',
    Extends: Endless.Application,

    vfunc_startup: function() {
        this.parent();

        let provider = new Gtk.CssProvider();
        provider.load_from_data(CSS);
        Gtk.StyleContext.add_provider_for_screen(
            Gdk.Screen.get_default(), provider,
            Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);

        this._label1 = new Gtk.Label({ label: "Should be 5em, or 60 pixels but scale with window" });
        this._test_window = new Endless.Window({
            application: this,
            font_scaling_active: true,
            font_scaling_min_font_size: 1
        });
        this._pm = this._test_window.get_page_manager();
        this._pm.add(this._label1);
        this._test_window.show_all();
    }
});

let app = new TestApplication({ application_id: TEST_APPLICATION_ID,
                                flags: 0 });
app.run(ARGV);