When you use relative, fixed, or absolute positioning, boxes can overlap
The float property allows you to take an element in normal flow and place it as far to the left or right of the containing element as possible
The clear property allows you to say that no element (within the same containing element) should touch the left or righthand sides of a box.
Resolution refers to the number of dots a screen shows per inch.
Because screen sizes and display resolutions vary so much, web designers often try to create pages of around 960-1000 pixels wide (since most users will be able to see designs this wide on their screens).
Fixed width layout designs do not change size as the user increases or decreases the size of their browser window. Measurements tend to be given in pixels.
Liquid layout designs stretch and contract as the user increases or decreases the size of their browser window. They tend to use percentages.
Grids help create professional and flexible designs
There are two ways to add multiple style sheets to a page:
Your HTML page can link to one style sheet and that stylesheet can use the @import rule to import other style sheets.
In the HTML you can use a separate element for each style sheet.