[css3-flexbox] fallback alignment of justify/distribute

Currently Flexbox has two modes for "justifying" items and lines within their containers:

   * align first and last box to container edges, space out the ones in between
   * space them all out evenly

Question A is, what happens when there's only one item? Should it center or start-align?
(The spec says different things for each one, which seems silly.)

Question B is, what happens when the content overflows? Should it center or start-align?

~fantasai