Programming

Progressive Enhancement Techniques 1: the HTML

    Craig Buckler
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    web layersIn my earlier articles, we discussed progressive enhancement and why it’s generally a better choice than graceful degradation. This is the first of a three-part series that illustrates how to build a simple tabbed box using progressive enhancement techniques.

    Web Page Layers

    Web pages are built in three layers:

    1. HTML defines the document content and structure
    2. CSS defines the layout and styling — it enhances the HTML
    3. JavaScript defines page functionality such as client-side form validation, animations and effects — it can enhance both the HTML and CSS

    HTML is the only layer you can rely on with any certainty. Most browsers will display something — even if you are providing XHTML 1.1 content to a device that only supports HTML 3.2.

    The majority of browsers offer CSS but the level of support varies. Users can disable CSS or specify their own stylesheets in preference to yours. Some browsers such as screen readers or Lynx could completely ignore your CSS.

    Finally, JavaScript is the least reliable technology layer. Most browsers support JavaScript but interpreter engines differ and many people disable the language for security reasons. The likelihood of a user having JavaScript will vary from site to site but, in general, up to 5% of visitors will not have scripting enabled.

    POSH Content

    POSH is an acronym for “Plain Old Semantic HTML”. The term was coined by the microformats community, but it’s a useful concept for all developers who strive to build well-structured, accessible, semantic and valid HTML:

    • use semantic elements and attributes, e.g. h1 for your main title, p for paragraphs, table for tables of data (not presentation), etc.
    • use semantic id and class names when necessary
    • use the simplest and least amount of HTML possible
    • validate your code.

    In the following example, we will build a tab box. The code is a simple client-side only widget, but the same concepts can be adapted throughout your development. Almost any control you can imagine will have a POSH equivalent.

    Building the Tab Box HTML

    At it’s most basic level, a tabbed box is a list of in-page links to associated content. The HTML structure could therefore be defined as follows:

    
<!-- tabs (links) -->
<ul class="tabs">
	<li><a href="#tab1">Tab 1</a></li>
	<li><a href="#tab2">Tab 2</a></li>
	<li><a href="#tab3">Tab 3</a></li>
</ul>

<!-- tab content -->
<div class="tabcontent">

	<div id="tab1">
		<h2>Tab 1 Content</h2>
		<p>This example uses progressive enhancement techniques to create a tabbed content box.</p>
	</div>
	
	<div id="tab2">
		<h2>Tab 2 Content</h2>
		<p>The control will work without CSS and JavaScript if necessary. Even a text-only browser such as Lynx will work.</p>
	</div>

	<div id="tab3">
		<h2>Tab 3 Content</h2>
		<p>For the best experience, users should use IE6+, Firefox 2+, Opera 9+, Safari 3+ or Chrome 1+ and have JavaScript enabled.</p>
	</div>

</div>

    This page should work in almost any web browser with support for HTML 4.01 or XHTML 1.0. Older browsers will continue to display the content even if the links do not work as expected.

    HTML only screenshot

    Note:
    Semantic HTML purists will point out that the code doesn’t follow POSH guidelines because it suffers from a DIV-itis. I agree — none of the DIVs are required for the content. They have been added because it will allow us to create a better CSS-only solution and simplify our JavaScript code.

    In a perfect world, your HTML would be simple and unconcerned about layout or functionality. Unfortunately, web development is not perfect and it’s often necessary to make compromises. Adding an extra tag or two for CSS or JavaScript hooks won’t destabilize the web; just don’t do it unnecessarily.

    In my next post, we add a the CSS layer to make it prettier and improve the usability without adversely affecting the functionality.

    Resource links:

    Other parts in this series:

    Related reading: