Using < strong > can be used in code to show that text is important, it basically puts the type in bold. The tag < em > can be used in a similar way to show importance. Em stands for emphasis and shows a subtly change. Typically em will be shown in italic by default.
The address tag is meant to be used to contain contact information for the page. This can contain a phone number or email, but it can also hold a physical address. This text is typically shown in italics so as to make it noticeable.
When making a background, if you do not choose a background color then the background will be transparent. To be sure a background has a white color or a color use the tag background-color in the body section of html. Padding can then be used to keep the type from touching the edges of the colored boxes.