Some brief tests on what compatibility mode Internet Explorer uses for various doctypes, focusing on how it's different from others.
Conclusion: The behaviour Internet Explorer 7 and 8b1 seems to follow is that only an XML declaraion is allowed above the doctype, otherwise it will use quirks mode.
NB: There is a full list of doctype behaviours at: hsivonen.iki.fi.
| Doctype | Moz | IE6 | IE7 | IE8b1 |
|---|---|---|---|---|
| HTML 4.01 Trans (No URL) | Quirks | Quirks | Quirks | Quirks |
| HTML 4.01 Trans | Standards | Standards | Standards | Standards |
| HTML 4.01 Strict | Standards | Standards | Standards | Standards |
| XHTML 1.0 Strict | Standards | Standards | Standards | Standards |
| XHTML + XML declaration | Standards | Quirks | Standards | Standards |
| XHTML + comment below | Standards | Standards | Standards | Standards |
| XHTML + comment above | Standards | Quirks | Quirks | Quirks |
| XHTML + XML declaration + comment above |
Standards | Quirks | Quirks | Quirks |
| XHTML + XML declaration + comment below |
Standards | Quirks | Quirks | Quirks |
| HTML5 | Standards | Standards | Standards |