<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
The DOCTYPE declaration tells the browser which version of HTML or XHTML is being used on the Web page. If you take a closer look at the DOCTYPE declaration, you'll notice it is XHTML 1.0 Transitional. The transitional part has nothing to do with XHTML per se. It is actually part of both HTML and XHTML and it relates to conformance levels. XHTML 1.0 and HTML 4.01 provide three conformance levels - strict, transitional, and frameset. Framesets are seldom used today due to problems with bookmarking and search engines. So frameset conformance is of little interest anymore. Transitional is a compatibility mode to make upgrading from prior specifications - such as HTML 3.0 - easier for existing pages. Strict is the way that the HTML/XHTML specifications were meant to be used. Strict disallows certain tags (e.g., <center>, <font>) and attributes (e.g., align, bgcolor, background) that are valid at transitional conformance. Disallowing these tags and attributes enforces better separation of structure from presentation in markup. Structure identifies the major building blocks of your page, such as a sidebar, breadcrumb trail, main heading, or article. Presentation is about how these building blocks should be rendered. For example, identifying a certain <div> tag as being the copyright notice is about structure. Noting that the <div> tag should be rendered in a x-small italic Helvetica font is about presentation. (See Roger Johansson's blog post, entitled Transitional vs. Strict Markup for a good discussion on the topic.)
So why does strict conformance disallow presentation-related tags/attributes in HTML/XHTML? It is because we now have a better mechanism for expressing presentation details... Cascading Style Sheets (CSS).
Let's Talk About CSS, Baby!
CSS or Cascading Style Sheets is a technology designed specifically to express presentation details about your Web page. Just like with HTML, I'm going to assume that you've had some exposure to CSS already. If you haven't, w3schools.com has a series of CSS Tutorials to introduce you to the technology.
Just like HTML, CSS evolved over time with CSS1, CSS2, and CSS 2.1. The
CSS 2.1 specification is supported by the latest version of the major browsers including Firefox 3 and Internet Explorer 8. Unfortunately, widely used browsers such as Firefox 2.0, Internet Explorer 6, and Internet Explorer 7 do not have full CSS 2.1 support,
as evidenced by the
Acid2 Test from
The Web Standards Project Acid Tests. Although the Acid2 Test is not going to win any art prizes, it is quite gruelling for browsers as it uses a variety of advanced CSS techniques to compose the smiley face, shown below.
Even though commonly used browsers such as Firefox 2.0 and Internet Explorer 6/7 do not render the Acid2 Test correctly, they do render most common CSS features faithfully. So as long as your stylesheets use predominantly CSS1 and commonly used CSS2/CSS2.1 features, your Web pages should render consistently across current browsers. Let's turn our attention to some recommended practices for CSS.
External Stylesheets vs. Inline Styles/Stylesheets
Styles can be included inline with a tag using the style attribute, inline with the page using a <style> tag, or in an external file with a <link> tag.
<!-- Inline style -->
<!-- Inline stylesheet -->
<style media="all" type="text/css"> /* Styles go here */ </style>
<!-- External stylesheet -->
<link media="all" type="text/css" href="Site.css" />
It is a bad, though common, practice to litter your HTML/XHMTL pages with style attributes. The result is a mix of presentational with structural concerns in your Web pages. Not only does this increase the size of your Web pages, but consistent styling across the Web application is difficult as the same styles are repeated for each similar element. A small improvement can be made by moving the styling information to a stylesheet included in the <head> section via the <style> tag. Styles are no longer repeated within the page, but you still have style duplication across Web pages. Using either inline style attributes or stylesheets is not recommended. In either case, changing a site's theme or fixing an error in your CSS is very time-consuming, as every Web page in the application must be updated. More preferable are external stylesheets, so that a common set of styles can be applied uniformly throughout the Web application. This has the additional advantage of allowing the browser to cache style information rather than sending it on every page request.
<div class="fontarial fontsize14 textred">Error</div>
Rather than specifying the structure of the document, CSS classes have been used to specify the presentation of the information. This is not how CSS is meant to be used. The preferred way to define this page element would be to use a CSS class to express its purpose.
The stylesheet might still use a 14px red Arial font to display errors, but notice how the structure - or intent - of the page element is expressed rather than its visual presentation.
The screen and the printed page are very different beasts. Each has its own set of layout concerns and optimizations. We have all encountered Web pages that provide a link to a "print-optimized" Web page that removes headers, sidebars, and background images. What if you could just print your Web page and have the browser optimize it for printing? Guess what? You can! CSS includes the notion of media-specific stylesheets. The styles are only applied for the specified media.
<link media="all" type="text/css" href="Site.css" />
<link media="print" type="text/css" href="PrintOnly.css" />
<link media="screen" type="text/css" href="ScreenOnly.css" />
You can include common styles in media="all" and override them for specific media in the other stylesheets. There are also media types for Braille, screen readers (aka aural), and more.
Compatibility is Easier with Valid CSS
Just like with HTML/XHTML, cross-browser compatibility of a Web application can be improved by using valid CSS. Rather than memorizing CSS specifications, you can use tools such as the W3C CSS Validation Service or Visual Studio 2008's built-in CSS Validator to ensure that not only are you writing valid CSS, but that you are only using CSS features up to a particular level, such as CSS2, but not CSS2.1, features.
Remember the "C" in CSS
An oft-forgotten feature of CSS is the cascading part - styles defined on parent tags cascade to their children. This means that you can define the font and text color for the <body> tag rather than on each individual element. (Note that cascading doesn’t make sense for some properties and hence not all properties cascade. For example, cascading the height property doesn’t make sense as all children would have the same height as the parent, which is clearly impossible for a stack of more than one child. Appendix F of the CSS2 recommendation has a full list of properties and whether child elements inherit the property from their parent.) Relying on cascading reduces duplication in your CSS and makes creating custom styles easier, as you have fewer styles to update. ScrewTurn Wiki contains a lot of duplication in its CSS.
Why is XHTML 1.0 Strict Missing From Visual Studio?
When selecting a conformance level in Visual Studio, there is no option for XHTML 1.0 Strict, as shown below.
The reason is because the XHTML 1.1 specification eliminates the transitional and frameset conformance levels, thus implicitly only implementing strict conformance. According to the specification, its purpose is "to provide a consistent, forward-looking document type cleanly separated from the deprecated, legacy functionality of HTML 4 that was brought forward into the XHTML 1.0 document types". There are minor differences between XHTML 1.0 Strict and XHTML 1.1, but for all practical purposes the two are close enough to obviate the need for both options in Visual Studio Schema Validator.