HTML Just Passing Through

答案：XML's many applications include one traditionally handled by HTML, 
document source. Sometimes you want to include some HTML in your XML 
source; particularly when styling your XML with XSL to HTML, you may 
want some literal HTML in the output. In some cases, you'd just like to 
include some well-formed mixed-content HTML in your XML, and not worry 
too much about controlling its structure.

Writing explicit XSL rules for each HTML tag would be tedious and hard 
to maintain. This week, I'll present a quick shortcut that allows you 
to include arbitrary mixed-content HTML in your XSL stylesheet's output.

The XSL stylesheet's input includes some XML document, constrained by 
(that is, validated against) a particular DTD. To define the element 
containing the arbitrary HTML's content model, simply define content 
models for each of the HTML elements you want to use and their 
attribute lists. For example, define the <a?tag for hyperlinks like 
this:

?!ELEMENT a (#PCDATA)*?BR> 
?!ATTLIST a
href CDATA #IMPLIED
target CDATA #IMPLIED
name CDATA #IMPLIED?BR> 
(This example is simplified, since it allows only unformatted text 
inside the hyperlink text node.) Create a definition like this for each 
HTML tag you wish to use. After you've defined the tags, define a 
parameter entity that includes all of the HTML "pass-through" tags 
you've defined:

?!ENTITY % htmlpassthru
"a|i|b|code|br|tr|td|th|img|font|em"?BR> 
Anywhere in the DTD you want to include mixed-content HTML, use this 
parameter entity in the DTD. For example:

?!ELEMENT HtmlHelpText (#PCDATA|%htmlpassthru;)*?BR> 

Now for the stylesheet. Defining a separate rule for each possible 
HTML element within a HtmlHelpText element would be extremely tedious. 
Fortunately, a single rule can handle all such elements. Define an XSL 
rule that matches the tag name AND uses <xsl:copy-of?to copy the 
entire node (including attributes) to the output.

?!-- EXAMPLE XSL stylesheet --?BR> 
?xsl:template
match="a|i|b|code|br|tr|td|th|img|font|em"?BR> ?xsl:copy-of select="."/?BR> ?/xsl:template?BR> 
?!-- End EXAMPLE --?BR> 
Any nodes in the XML input tree with these tags will be copied to the 
output structure unmodified, attributes and all.

This technique is somewhat "quick-and-dirty". Choosing XHTML, which 
reformulates HTML as XML and brings XML's extensibility and structure 
control into the world of HTML, would be a better solution. But that's 
a topic for another newsletter.

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