IE 8 Document Compatibility

We can use Document Compatibility of IE 8 to choose the specific rendering mode that Internet Explorer uses to display your Web pages.

Document compatibility helps us to make our web page to have consistent look in various versions of IE. We can select rendering type by using meta tag within head tag as first element:

<meta http-equiv="X-UA-Compatible" content="IE=7" >

By using above tag, we can make the page to be rendered as IE 7 in IE 8 browser.
Other options are:

Emulate IE8 mode tells Internet Explorer to use the <!DOCTYPE> directive to determine how to render content. Standards mode directives are displayed in Internet Explorer 8 standards mode and quirks mode directives are displayed in IE5 mode. Unlike IE8 mode, Emulate IE8 mode respects the <!DOCTYPE> directive.

Emulate IE7 mode tells Internet Explorer to use the <!DOCTYPE> directive to determine how to render content. Standards mode directives are displayed in Internet Explorer 7 standards mode and quirks mode directives are displayed in IE5 mode. Unlike IE7 mode, Emulate IE7 mode respects the <!DOCTYPE> directive. For many Web sites, this is the preferred compatibility mode.

IE5 mode renders content as if it were displayed by Internet Explorer 7's quirks mode, which is very similar to the way content was displayed in Internet Explorer 5.

IE7 mode renders content as if it were displayed by Internet Explorer 7's standards mode, whether or not the page contains a <!DOCTYPE> directive.

IE8 mode provides the highest support available for industry standards, including the W3C Cascading Style Sheets Level 2.1 Specification  and the W3C Selectors API , and limited support for the W3C Cascading Style Sheets Level 3 Specification (Working Draft) .

Edge mode tells Internet Explorer to display content in the highest mode available. With Internet Explorer 8, this is equivalent to IE8 mode. If a (hypothetical) future release of Internet Explorer supported a higher compatibility mode, pages set to edge mode would appear in the highest mode supported by that version. Those same pages would still appear in IE8 mode when viewed with Internet Explorer 8.