We have quite a number of projects with responsive layout implemented on. It is necessary to check that behavior on various type of devices and screen sizes. There is a Firefox plugin named Web Developer Toolbar which can help you on this. If you dont have this plugin already, go for it right now.
A hidden region is a new feature in drupal 7. I was modifying a drupal 7 theme. In the .info file I found two regions - 'page_top' and 'page_bottom'. I wanted to assign some blocks to those regions. But surprisingly I couldn’t find them in block administration area. After some searching I came to know about the feature – hidden region. It's a way to define regions that are available for modules but are not available in the blocks administration area.
Internet explorer 6, 7 and 8 has some weird behavior for iFrame rendering. They tend to not respecting border and background styles provided by CSS styles. So, you might find something odd looking in lower IE versions like this: