Transitional vs. Strict Markup

When promoting web standards, standardistas often talk about XHTML as being more strict than HTML. In a sense it is, since it requires that all elements are properly closed and that attribute values are quoted. But there are two flavours of XHTML 1.0 (three if you count the Frameset DOCTYPE, which is outside the scope of this article), defined by the Transitional and Strict DOCTYPEs. And HTML 4.01 also comes in those flavours.

The names reveal what they are about: Transitional DOCTYPEs are meant for those making the transition from older markup to modern ways. Strict DOCTYPEs are actually the default – the way HTML 4.01 and XHTML 1.0 were constructed to be used.

A Transitional DOCTYPE may be used when you have a lot of legacy markup that cannot easily be converted to comply with a Strict DOCTYPE. But Strict is what you should be aiming for. It encourages, and in some cases enforces, the separation of structure and presentation, moving the presentational aspects from markup to CSS. From the HTML 4 Document Type Definition:This is HTML 4.01 Strict DTD, which excludes the presentation attributes and elements that W3C expects to phase out as support for style sheets matures. Authors should use the Strict DTD when possible, but may use the Transitional DTD when support for presentation attribute and elements is required.

An additional benefit of using a Strict DOCTYPE is that doing so will ensure that browsers use their strictest, most standards compliant rendering modes.

Tommy Olsson provides a good summary of the benefits of using Strict over Transitional in Ten questions for Tommy Olsson at Web Standards Group:In my opinion, using a Strict DTD, either HTML 4.01 Strict or XHTML 1.0 Strict, is far more important for the quality of the future web than whether or not there is an X in front of the name. The Strict DTD promotes a separation of structure and presentation, which makes a site so much easier to maintain.

For those looking to start using web standards and valid, semantic markup, it is important to understand the difference between Transitional and Strict DOCTYPEs. For complete listings of the differences between Transitional and Strict DOCTYPEs, see XHTML: Differences between Strict & Transitional, Comparison of Strict and Transitional XHTML, and XHTML1.0 Element Attributes by DTD.

Some of the differences are more likely than others to cause problems for developers moving from a Transitional DOCTYPE to a Strict one, and I’d like to mention a few of those.

Elements that are not allowed in Strict DOCTYPEs

• center
• font
• iframe
• strike
• u

Attributes not allowed in Strict DOCTYPEs

• align (allowed on elements related to tables: col, colgroup, tbody, td, tfoot, th, thead, and tr)
• language
• background
• bgcolor
• border (allowed on table)
• height (allowed on img and object)
• hspace
• name (allowed in HTML 4.01 Strict, not allowed on form and img in XHTML 1.0 Strict)
• noshade
• nowrap
• target
• text, link, vlink, and alink
• vspace
• width (allowed on img, object, table, col, and colgroup)

HTML5 is a language for structuring and presenting content for the World Wide Web, a core technology of the Internet. It is the latest revision of the HTML standard (originally created in 1990) and currently remains under development. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers etc.).

Following its immediate predecessors HTML 4.01 and XHTML 1.1, HTML5 is a response to the observation that the HTML and XHTML in common use on the World Wide Web is a mixture of features introduced by various specifications, along with those introduced by software products such as web browsers, those established by common practice, and the many syntax errors in existing web documents. It is also an attempt to define a single markup language that can be written in either HTML or XHTML syntax. It includes detailed processing models to encourage more interoperable implementations; it extends, improves and rationalises the markup available for documents, and introduces markup and APIs for complex web applications. In particular, HTML5 adds many new syntactical features. These include the and elements, as well as the integration of SVG content. These features are designed to make it easy to include and handle multimedia and graphical content on the web without having to resort to proprietary plugins and APIs. Other new elements, such as are designed to enrich the semantic content of documents. New attributes have been introduced for the same purpose, while some elements and attributes have been removed. Some elements, such as , and have been changed, redefined or standardised. The APIs and DOM are no longer afterthoughts, but are fundamental parts of the HTML5 specification. HTML5 also defines in some detail the required processing for invalid documents, so that syntax errors will be treated uniformly by all conforming browsers and other user agents