Neither the HTML 4. The document type declaration is placed in the document before the start tag of the root element. This is what you should use. Please be sure to test your page in the latest versions of the top browsers. This gives you the Almost Standards mode.
Willfully designing for the Quirks mode will come and haunt you, your coworkers or your successors in the future. If you frustrated by the differences between old IE versions and still need to support them due to client requirements, it is better to apply specific hacks for legacy versions using conditional comments than seek commonality in the Quirks mode.
Please see the Addendum below. It was announced on A List Apart that IE8 would use a meta element-based mode switch in addition to doctype being a factor in the mode choice.
IE8 has four modes: IE 5. IE9 has seven modes: IE 5. IE10 has eleven modes: IE 5. The choice of mode depends on data from various sources: doctype, a meta element, an HTTP header, periodically downloaded data from Microsoft, the intranet zone, settings made by the user, settings made by an intranet administrator, the mode of the frame parent if any and a UI button togglable by the user. With other apps that embed the engine, the mode also depends on the embedding application.
The lucky thing is that IE8 and IE9 use doctype sniffing roughly like other browsers and IE10 uses doctype sniffing exactly like other browsers if all the following points are true:.
The IE7 emulation is called Compatibility View. Contrast with the chart for other browsers as PDF. There is also a unified chart of IE 5. Worse, an intranet admin may do this.
Also, Microsoft may have put the entire domain you use e. Google Chrome Frame is a combination of a browser extension and browser plug-in for IE 6, 7, 8 and 9 that adds the engine of Google Chrome into the user interface shell of IE using the networking stack that IE uses.
After installation, IE behaves normally by default. Google Chrome Frame was discontinued years ago. You should not try to accommodate it. It is mentioned here only for historical completeness. Doctype sniffing is a tag soup solution to a tag soup problem. Doctype sniffing was devised after the HTML 4 and CSS2 specs had been written as a heuristic way to distinguish legacy documents from documents whose authors might expect conforming behavior.
Sometimes it is suggested that doctype sniffing be used on the XML side as well for dispatching to different handlers, for recognizing the vocabulary in use, or for activating features.
This is a bad idea. Dispatching and vocabulary recognition should be based on namespaces and feature activation should be based on explicit processing instructions or elements. The whole concept of well-formedness was introduced to allow DTDless parsing of XML and, by extension, doctypeless documents.
In formal terms, if two XML documents have the same canonical form and an app treats them differently and the difference is not due to opting not to process external entities , chances are the app is broken. In practical terms, if two XML documents cause the same content to be reported qnames ignored to SAX2 ContentHandler and an app treats the documents differently, chances are the app is broken. Considering that as a Web author you cannot trust that everyone parsing your pages uses an XML processor that resolves external entities even if some browsers appear to do so because they map certain public ids to an abridged DTD for entity definitions , inserting a doctype in XML intended for the Web is mostly pointless and often done out of a cargo cultish habit.
It relates mainly to IE 7. Other browsers may have a Quirks Mode that does simulate old versions of IE to the same extent. The following simple images demonstrate one of the many differences between Quirks Mode and Standards Mode on Internet Explorer, namely the box model. In Standards Mode, it specifies the width of the content of the element, so that the total width of the box 12em plus the widths of the borders.
On Firefox, the correct box model is applied in both modes. A form that should have yellow background. If the form has a border set for it, it may be drawn incorrectly, excluding some part of the form. There is a simple separate test page for this IE 7 bug. This bug has been observed on Opera, too. Whether the bug is triggered seems to depend on how the content of the form starts, in terms of markup. This bug appears in both Standards Mode and in Quirks Mode.
In IE 8 and IE 9, this bug still exists, in different manifestations. Existing CSS would start to show odd side effects if it were suddenly interpreted in the correct way. So moving closer to standards compliance would cause problems. On the other hand, not moving closer to standards compliance would perpetuate the general confusion of the Browser Wars Era. In other words, all browsers needed two modes: quirks mode for the old rules, strict mode for the standard.
IE 5 Windows, as well as older browsers like Netscape 4, are permanently locked in quirks mode. Note that your page does not have to validate according to the chosen doctype, the mere presence of the doctype tag is enough to trigger strict mode. On this site I use this doctype in most pages.
But the browser vendors were not to be denied: browser after browser implemented doctype switching, and nowadays all modern browsers support it.
Nonetheless browser vendors, Mozilla especially, thought this was such a confusing situation that they introduced "almost strict mode". This was defined as strict mode, but with images continuing to be blocks, and not inline elements.
Most common doctypes, including the one I use, trigger almost strict mode. The treatment of images is by far the most important difference between almost strict mode and really strict mode.
In IE 6 Windows, Microsoft implemented one extra rule: if a doctype that triggers strict mode is preceded by an xml prolog, the page shows in quirks mode. This was done to allow web developers to achieve valid pages which require a doctype but nonetheless stay in quirks mode. This is the xml prolog. You should put it on the very first line of your document, before the doctype. What, exactly, are the differences between the two modes? There are a few more differences that can trip up the unwary web developer.
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