ITEC 325 2014spring ibarland

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lect10-xhtml-intro
xhtml intro

xhtml : Even though original HTML doesn't require the following, they are best practices, and they are required for this class:

• Tag names are case-sensitive (with a preference for lower-case and camelCase).
• quote all attribute-values, even if it looks numeric;
• well-formed (duh): each open-tag must be eventually followed by a corresponding close-tag, and if a tag's body contains another open-tag it must also contain the corresponding close-tag.
  • Bad: This is <em>not <strong>at all</em> legal</strong> XML.
  • Good:This is <em>actually <strong>very</strong> good</em> practice.
  Another way of viewing this: The open/close tags must correspond to a tree.

  The most common (non-X)HTML example using improperly nested tags is <p>, which (in HTML) doesn't need to be closed; the browser auto-closes the tag when it reaches the next <p> (or more precisely: when it reaches the next block-level open-tag, or a close-tag which is terminating the enclosing block, e.g. if the paragraph was already inside an itemized list).

  Browsers tend to go to great lengths to try to make sense of tag soup; just because a page looks fine in your browser doesn't mean it's legal. The result is that many people learned their HTML by doing a show-source on bad, illegal examples, and then propagating that bad HTML, and never realizing it because most browsers still did something reasonable (although the details of how a browser handled bad HTML are of course entirely non-standard).

• Self-closing tags:
  • <br></br>— fine, but long-winded.
  • <br/>— self-closing tag, allowed by XML.
  • <br />— Best practice¹: used for backwards compatability. The original HTML required a space before the /, and it's conceivable (if unlikely) that a modern browser will insist on using the HTML standard
  • <br> </br>— (note the space inside the tag) this version is not equivalent (and, not valid HTML) — spaces are part of your XML text, even if an HTML browser chooses often ignore them for layout purposes.

  Important: In HTML (and enforced by most modern browsers), you can only use the self-closing tag, for those tags which never are allowed a body. For tags that may or may not have a body (notably: script), you cannot use the self-closing version even when you want an empty body. There is no good reason for this!

  • <hr />— fine; hr never has a body.
  • <img src="foo.jpg" />— fine; img never has a body.
  • <script type="text/javascript" src="foo.jpg"></script>— fine.
  • <script type="text/javascript" >callSomeFunction();</script>— fine.
  • <script type="text/javascript" src="foo.jpg" />— BAD; a browser may ignore the tag entirely!, which is an annoyingly difficult bug to track down.

See also: xhtml.com/en/future/x-html-5-versus-xhtml-2/.

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©2014, Ian Barland, Radford University Last modified 2014.Feb.13 (Thu)

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