While Opera is well capable of showing standards-compliant pages correctly, not all web pages are compliant. To be able to both display all standards-compliant pages and the other pages as well, two different display modes have been added in Opera 9.0: Standards Mode (with IE known as Standards-compliance Mode) and Quirks Mode (essentially an Internet Explorer compatible mode). Which of the two modes is selected depends on which Document Type Declaration (DOCTYPE) the page is using.
Internet Explorer (from 6.0 on for Windows and 5.0 on for Mac) and Mozilla/Netscape (from versions 0.6 and 6.0 respectively) also use DOCTYPE switches. Recent Netscapes have a third mode, an "Almost Standards Mode" that is almost a Standards Mode. Opera 9 uses the same set of rules as IE6 for Windows to determine whether to enter Standards Mode or Quirks Mode.
|Document type||Example Document Type Declaration||URL present||URL absent|
|HTML (No version)||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML//EN" "http://www.w3.org/TR/html/loose.dtd">||Quirks||Quirks|
|HTML 2.0||<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">||Quirks||Quirks|
|HTML 3.0||<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 3.0//EN//">||Quirks||Quirks|
|HTML 3.2||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">||Quirks||Quirks|
|HTML 4.0||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">||Standards||Standards|
|HTML 4.0 Frameset||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN" "http://www.w3.org/TR/REC-html40/frameset.dtd">||Standards ||Quirks|
|HTML 4.0 Transitional||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">||Standards ||Quirks|
|HTML 4.0 Strict||<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">||Standards||Standards|
|XHTML||<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN" "xhtml-basic10.dtd">||Standards ||Standards |
|WML 2.0||<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD WML 2.0//EN" "http://www.wapforum.org/DTD/wml20.dtd">||Standards ||Standards |
 Triggers Quirks mode for Netscape
 When XHTML is delivered as
text/html and includes the XML declaration, Quirks mode is triggered for IE and Opera 7.0-7.03. From Opera 7.1 on, the XML declaration, PIs, and comments are ignored for determining display mode. When delivered as XML, both Opera and IE use Standards mode.
 Only Opera recognizes the WML doctypes
document.compatMode. When a page is rendered according to Quirks Mode, this property is set to "
BackCompat", in Standards Mode it is set to "
Quirks Mode enables Opera to display existing invalid pages. If you make pages from scratch, your best option is to make them standards-compliant and use a DOCTYPE to ensure Standards Mode, but see "Incompatible renderings" below. You may have to add workarounds.
If you have existing pages and they are only tested with Netscape 4 and IE, going for Quirks Mode may be a safer bet initially. If after testing a page looks good in these browsers and in Opera in Standards Mode as well, you may safely change to Standards Mode.
There will not be a complete list of rendering differences of Opera in Quirks Mode and in Standards Mode. This will continously change with whatever standards divergences are common at any given time. The rendering in Standards Mode will always follow the standards. Below are some of the most important differences. They are mostly in one of two categories:
Unit-less numbers in CSS (e.g.
"width: 300") are interpreted as pixels in Quirks Mode and ignored in Standards Mode.
Spaces between the number and unit (e.g.
"width: 45 em") is not valid CSS. Opera allows this in Quirks Mode and ignores properties with invalid lengths in Standards Mode.
A class or id must begin with a letter or an underscore, not with a number (e.g.
<div class="2">). Opera ignores such selectors in Standards Mode, but allows class names starting with a number in Quirks Mode.
height' attribute in table rows (7.0-7.11)
The HTML '
tr' element doesn't have a '
height' attribute. Opera 7.0-7.11 only allowed this attribute in Quirks Mode, Opera 7.2 and later also allows '
height' in Standards Mode.
In Quirks Mode, Opera will expand the block element to fit the content. In Standards Mode, Opera honours the height and applies the style overflow:visible if nothing else is specified.
A bug IE/Win and Netscape 4 share is that CSS properties aren't inherited into tables. Opera emulates this bug in Quirks Mode, but not in Standards Mode.
The CSS '
width' property should be ignored for inline elements. In Quirks Mode assigning a width to an inline element changes the display type to inline-block.
In Internet Explorer/Windows (except IE6 in standards compliant mode)
font-size: small is interpreted as the initial font size. Opera in Quirks Mode emulates this behavior, but uses
font-size: medium in Standards Mode. This means that the absolute font size keywords are "one size larger" in Quirks Mode.
In Strict mode,
class="CLASS" is different from
class="class", and the CSS selector
*.CLASS will only select the former. In Quirks Mode, case doesn't matter and
*.CLASS will match both. Likewise,
id="id" is different from
id="ID" in Standards Mode, but not in Quirks Mode.
The CSS '
width' property specifies content width. In IE/Win 3.0-5.5 it specifies content width + padding width + border. The same applies to the '
height' property. Opera in Quirks Mode emulates this behavior. Opera and IE/Mac, but unfortunately not IE/Win, support the
box-sizing CSS property proposed for CSS 3. Netscape and Mozilla supports the equivalent
In quirks mode, the CSS properties '
margin-left' and '
margin-right' is subtracted from the width of containing box when computing percentage widths, not so in Standards Mode.
Normally the DOCTYPE Switch determines which mode is used, but you can set Opera to always use Standards Mode or always use Quirks Mode by setting one of these lines in the opera6.ini file:
If you design Web pages, setting Opera to always using Standards Mode will help you see how a page will look like in a browser not supporting DOCTYPE switching.
WML 2.0 uses a special form for DOCTYPE switching. In most respects WML 2.0 is XHTML Basic with some WML extensions and it uses the same namespace as XHTML Basic. In a few cases the WML behavior differs from XHTML, such as the handling of WML variables and $ escaping. See the WML 2.0 Specification for details.
Need help? Hit F1 anytime while using Opera to access our online help files, or go here.