Open an html page in a default browser from an add-in

You can open a new browser window from an Office Add-in via JavaScript: simply

window.open("your-url.com");

Alternatively, if you want the browsing experience to be more in-line, you can use the dialog API:

Office.context.ui.displayDialogAsync(url,
    { height: 75, width: 80, requireHTTPS: true });

See https://github.com/OfficeDev/Office-Add-in-UX-Design-Patterns-Code/tree/master/templates/feedback/office-store for a full example.

~ Michael Zlatkovsky, Developer on Office Extensibility Team, MSFT