Modern browsers have at least three modes in which they can display content:
In standards-compliant mode, the browser does its best to display the page according to the latest CSS standards.
In almost standards mode the browser will treat everything the same as in standards mode except for the placement of images inside of table cells. Images will align with the very bottom of a table cell instead of text baseline that strict standards mode dictates.
In quirks mode, the browser displays the page as if it were an older version of the same browser. The purpose of quirks mode is to make pages that were designed for the old browsers display the same in the newer browsers.
The correct DOCTYPE declaration at the top of the HTML page sets the mode modern browsers will display the page in. Wikipedia provides an excellent guide to the different DOCTYPE declarations and the mode effect it has on browsers at http://en.wikipedia.org/wiki/Quirks_mode.
Setting the DOCTYPE gives the page designer control over how the browser agent will interpret and render HTML of the page.
When designing your pages, we recommend you use the following strategy:
Depending on your need to support other browsers, you may wish to repeat steps two and three on those browsers as well.