Span and div
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In HTML (including XHTML), the span and div elements are used where parts of a document cannot be semantically described by other HTML elements.
Most HTML elements carry semantic meaning – i.e. the element describes, and can be made to function according to, the type of data contained within. For example, a p element should contain a paragraph of text, while an h1 element should contain the highest-level header of the page; user agents should distinguish them accordingly. However, as span and div have no innate semantic meaning besides the logical grouping of the content, they can be used to specify non-standard presentation or behaviour without superfluous semantic meaning.
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[edit] Differences and default behavior
There is one difference between div and span. In standard HTML, a div is a block-level element whereas a span is an inline element. The div block visually isolates a section of a document on the page, in the same way as a paragraph (see example). The span element contains a piece of information inline with the surrounding text (see example). In practice, even this feature can be changed by the use of CSS.
[edit] Practical usage
span and div elements are used purely to imply a logical grouping of enclosed elements.
When they are labeled with class or id attributes, span and div elements can denote types of information otherwise indescribable with HTML. For example, <div id="byline">Fred Smith</div> may be used to indicate the author's name in a document, and <span class="date">21st Jan 2006</span> may be used specifically to indicate a date.
There are three main reasons to use spans and divs with class or id attributes:
[edit] Styling with CSS
Perhaps the most common use of span and div elements in order to apply class or id attributes is when this is done in conjunction with Cascading Style Sheets (CSS) to apply layout, typographic, color and other presentation attributes to the page's content. CSS does not just apply to visual styling: when spoken out loud by a voice browser, CSS styling can affect speech-rate, stress, richness and even position within a stereophonic image.
For these reasons, and for compatibility with the concepts of the semantic web, discussed below, attributes attached to elements within any HTML should describe their semantic purpose, rather than merely their intended display properties in one particular medium. For example, <span class="red small">password too short</span> is semantically meaningless, whereas <span class="warning">password too short</span> is much more useful. By the correct use of CSS, on the screen 'warnings' may be rendered in a red, small font, but when printed out, they may be omitted, as by then it is too late to do anything about them. Perhaps when spoken they should be given extra stress, and a small reduction in speech-rate. The second example is semantic markup, rather than merely presentational, but serves both purposes when combined with CSS.
[edit] Semantic clarity
This kind of grouping and labeling of parts of the page content might be introduced purely to make the page more semantically meaningful in general terms. It is impossible to say how and in what ways the World Wide Web will develop in years and decades to come. Web pages designed today may still be in use when information systems that we cannot yet imagine are trawling, processing, and classifying the web. Even today's search engines such as Google and others are using proprietary information processing algorithms of considerable complexity.
The World Wide Web Consortium (W3C) has for some years been running a major Semantic Web project designed to make the whole web increasingly useful and meaningful to today's and the future's information systems.
During the page design process, the designer has a clear idea of exactly the purpose and meaning of each element and sub-element of the content. If possible, and if standard HTML elements exist that express that meaning, they should be used. If not, there is no better time to encapsulate the meaning and purpose in a span or div element with appropriate class or id attributes. If nothing more, doing so will help future editors to maintain the markup.
The Microformats movement is an attempt to build on this idea of semantic classes. For example, microformats-aware software might automatically find an element like <span class="tel">123-456-7890</span> and allow for automatic dialing of the telephone number.
[edit] Access from code
Once the HTML or XHTML markup is delivered to a page-visitor's client browser, there is a chance that client-side code will need to navigate the internal structure (or Document Object Model) of the web page. The most common reason for this is that the page is delivered with client-side JavaScript that will produce on-going dynamic behavior after the page is rendered. For example, if rolling the mouse over a 'Buy now' link is meant to make the price, elsewhere on the page, become emphasized, JavaScript code can do this, but JavaScript needs to identify the price element, wherever it is in the markup, in order to affect it. The following markup would suffice: <div id="price">$45.99</div>. Another example is the Ajax programming technique, where, for example, clicking a hypertext link may cause JavaScript code to retrieve the text for a new price quotation to display in place of the current one within the page, without re-loading the whole page. When the new text arrives back from the server, the JavaScript must identify the exact region on the page to replace with the new information.
Less common, but just as important examples of code gaining access to final web pages, and having to use span and div elements' class or id attributes to navigate within the page include the use of automatic testing tools. On dynamically generated HTML, this may include the use of automatic page testing tools such as HttpUnit, a member of the xUnit family, and load or stress testing tools such as JMeter when applied to form-driven web sites.
[edit] Overuse
The judicious use of div and span is a vital part of HTML and XHTML markup. However, the overuse of these elements, sometimes called divitis (a common mistake of beginners), is itself a minor form of tag soup.
For example, when structurally and semantically a series of items need an outer, containing element and then further containers for each item, then there are various list structures available in HTML, one of which may be preferable to a homemade mixture of divs and spans.
For example, this...
<ul class="menu"> <li>Main page</li> <li>Contents</li> <li>Help</li> </ul>
...is usually preferable to this:
<div class="menu"> <span>Main page</span> <span>Contents</span> <span>Help</span> </div>
Other examples of the semantic use of HTML rather than divs and spans include the use of fieldset elements to divide up a web form, the use of legend elements to identify such divisions and the use of labels to identify form input elements rather than divs, spans or table elements used for such purposes.
