The visual differences illustrated above arise from the following implementation differences in a browser such as Internet Explorer:. In standards mode the CSS width setting applied to the div does not absorb any widths set for padding and border settings, whereas in quirks mode it does — which is why the large box is wider in the left-most standards picture. In quirks mode the table has not inherited the font size setting from the body element, so the text looks larger.
It is generally a good idea to always serve your pages in standards mode — ie. There is one aspect of using DOCTYPEs that is critically important for character encoding declarations and for predictable styling results. This would make the top of the above file look like this the XML declaration is highlighted in red italics :. If Internet Explorer 6 users still count for a significant proportion of your intended audience, this may be an issue.
If you want to ensure that your pages are rendered in the same way as on all other standards-compliant browsers, you need to think carefully about how you deal with this. Obviously, if your document contains no constructs that are affected by the difference between standards vs. If, on the other hand, that is not the case, you will have to add workarounds to your CSS to overcome the differences, or omit the XML declaration.
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The Document Type Definition DTD, which is referenced by the doctype declaration then defines which elements, attributes, and character entities exist in the language and where those elements may be placed. I'll explain those benefits later in this article, but first I'd like to debunk some of the false benefits you may have heard. Let's clear up some of them at a glance with details and other pitfalls provided later :.
Semantic markup and separation of content and presentation is absolutely possible in HTML, and with equal ease. In terms of semantics, HTML 4. XML parsers do not typically check documents for validity. They only check them for well-formedness, which is a separate concept. If you leave out a required element, use deprecated or nonstandard elements or attributes, or put an element somewhere it isn't allowed, the XML parser will provide no indication of the error, and the browser will have to silently deal with the error like HTML parsers do.
HTML is not deprecated and is not being phased out at this time. HTML5 is it. XHTML 1. A simple XSL transformation will not be sufficient in most cases, because some semantics won't translate properly. HTML 4. A valid HTML 4. XHTML does not have good browser support.
But when you do, Internet Explorer and a number of other user agents will choke on it and won't display a page at all. Even when handled as XHTML, the supporting browsers have a number of additional bugs, which I'll also discuss in this article. The only major additional complexity of HTML that is well supported is tag omission, but most browsers use hardcoded rules specific to HTML in order to cheat through that with minimal performance impact.
The browser can lose some minor shorthand logic with XML, but it now has to use extra logic to confirm that the document is well-formed. And either way, download speed is usually the bottleneck when it comes to document parsing. The users won't notice any speed difference. They will handle the document as HTML and you will have no extensibility benefit.
If you prefer using lower-case tag and attribute names, you can do so in HTML. If you prefer having quotes around all attribute values, you may do so in HTML. If you prefer making sure all of your non-empty elements have end tags, you may use end tags in HTML, too.
In fact, these are considered best practice principles in HTML. Although HTML's syntax allowed for a lot of shorthand markup and other flexibility, it proved too difficult to write a correct and fully-featured parser for it, since a truly correct parser would have to support the entire SGML standard. XML was designed to eliminate these extra features and restrict documents to a tight set of rules that are more straight-forward for user agents to implement.
Most of the features that would make HTML more difficult to write a parser for, such as custom SGML declarations, additional marked sections, and most of the shorthand constructs, have negligible use on the Web anyway and generally have poor or absent support in major web browsers. The most significant difference is XML's lack of support for omitted start and end tags, which in theory could amount to complicated logic in HTML for elements not defined as empty.
Even still, most browsers don't bother to implement real DTD-based parsing logic, so it isn't quite so complicated in practice. In hopes of eliminating some error handling logic, XML user agents are told to not be flexible with errors: if a user agent comes upon a problem in the XML document, it will simply give up trying to read it.
While user agents are supposed to fail on any page that isn't well-formed in other words, one that doesn't follow the generic XML grammar rules , they do not have to fail on a page that is well-formed but invalid. For example, although it is invalid to have a span element as an immediate child of the body element, most XML-supporting web browsers won't provide indication of the error because the page is still well-formed — that is, the DTD is violated, but not the fundamental rules of XML itself.
There is some worry that people may rely too heavily on the well-formedness checker and forget to also check for validity, which could lead to a higher occurrence of invalid pages even among the otherwise standards-conscious developers. Despite popular assumption, even if an XML page is perfectly valid according to some validators, it still might not be well-formed.
Well-formedness involves some requirements not present in the classic SGML definition of validity.
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