When switching between the different Device Channels you can persist the choice using the DeviceChannel cookie.
When referencing CSS files in HTML files used with Design Manager CSS media queries are removed from the link tag.
Mavention Insert Table of Contents is now available as an App for SharePoint.
Design Manager allows you to quickly implement your user experience in SharePoint but if you want to build a great public-facing website you should build the Master Page and Page Layouts yourself instead.
On your public-facing website use the XML Sitemap and Robots.txt file to optimize your website for Internet search engines.