Backbone is for front end development, it allows you to link your data objects to a visual layout, its sort of a analogy of its name, its a backbone for you UI build.

Node uses asynchronous javascript from server HTTP requests and can also be used for many other server implementations, but chiefly its a back end server mechanism.

Angular seems really over the top in my opinion, if your designing responsive layouts then your should rely on progressive design techniques and keep to the use of CSS3 or eve CSS frameworks such as Less.js

HTML5 really is piss easy, its only the javascript implementation that takes some getting used to, that is ofcourse if your heading down the canvas route, or interactive data storage (drag and drop file uploading, local storage api etc.

Browser targetting really is a demographic deal, if your looking at targeting corporate markets then I'm afraid you really are going to have to optimise for IE6, which is why I would probably avoid HTML5/CSS3 since it has piss poor support. However if your building a back end system you can insist to your clients that they must use a certain level of browser for security concerns. But generally, the market share for IE has dropped considerably, and personally I don't even bother with it any more.

I've been doing web development since I was 14 and go to regular JS/jQuery conferences, it gets pretty tedious after a while, which is why I'm trying out Android at the moment