Defining the relative vertical height of an element
Sometimes we want to define a vertical height of an element.
It is simple, using fixed values (not pixels), but it's more complicated when using relative heights (percentage).
This problem is often encountered when using background images to create a border with a certain style to an element.
Solution
To set a vertical height on an item, it is necessary that every parent element have a defined height too.
We must define a height tags to html, body, and all tags containing the element must have a certain height.
This gives for example:
html,body,div#body,div#all_my_elements{height:100%} 
Acer vertical line on screen
