Demonstrate auto side margins (this also demonstrates the width property as applied to a block frame):
This block has width=auto therefore no auto side margins will apply
This block has width=50% therefore auto side margins will apply. The left margin is auto therefore this should be right-aligned.
This block has width=50% therefore auto side margins will apply. Both margins are auto therefore this should be centered.
This block has width=50% therefore auto side margins will apply. The right margin is auto therefore this should be left-aligned.