elm232

They're not though. The author worked for Twitter when he released the first version of Bootstrap, but Twitter never actually used it in production. Twitter doesn't promote it anymore either; BS3 got moved to twbs/bootstrap on GitHub, and the new marketing site doesn't call it Twitter Bootstrap anymore.

ZURB doesn't have a huge consumer product, but they're a pretty big name in design, and they have half a dozen people who regularly work on Foundation/Foundation support.

Check out Ink's block-grid (http://zurb.com/ink/docs.php#block-grid). It's a lifesaver for working with Gmail mobile. Alternatively, if you don't need Outlook support, ZURB's original responsive templates support Gmail mobile quite well (http://zurb.com/playground/responsive-email-templates).

Backing up what ricefield said, responsive design is making something that works well in many different viewing contexts (desktop, mobile, tablet, smart TV, etc). http://zurb.com/word/responsive-web-design

Placehold.it might have been down. They're great for development, but a lot of people were probably opening the templates and sucking up their bandwidth yesterday.