IE7について

1. Be aware that the HTML element has a default border of 1.
2. Windows Internet Explorer 7 and greater: Fixed positioning is available through strict mode.
   
   Internet Explorer 6 and greater: It is recommended that your !DOCTYPE be set to strict mode to enable rendering compliance with the W3C specification for Cascading Style Sheets, Level 2.1 (CSS2.1) .

   Fixed Positioning

   Beginning in Internet Explorer 7, web developers can use fixed positioning, a subcategory of absolute positioning. Similar to the containing boxes of absolute positioning, fixed position elements are independent. When applied, they are not relative to preexisting parent or child elements. The distinction involves element placement. Rather than localization on the body of a page (as with absolute positioning), the contents of a fixed positioned element flow within the given dimensions of the browser window (also known as the viewport).

   
   


3. By default, a positioned element always has a higher z-coordinate than its parent element so that it will always be on top of its parent element.