HTML and CSS Reference
In-Depth Information
15.6. Conditional Sections
As we mentioned earlier in this chapter, XML lets you include or ignore
whole sections of your DTD, so you can tailor the language for alternative
uses. The HTML DTD, for instance, defines transitional, strict, and frame-
based versions of the language. DTD authors can select the portions of
the DTD they plan to include or ignore by using XML conditional direct-
ives:
<![INCLUDE [
...
any XML content
...
]]>
or:
<![IGNORE [
...
any XML content
...
]]>
The XML processor either includes or ignores the contents, respectively.
Conditional sections may be nested, with the caveat that all sections con-
tained within an ignored section are ignored, even if they are set to be
included.
You rarely see a DTD with the
INCLUDE
and
IGNORE
keywords spelled out.
Instead, you see parameter entities that document why the section is
being included or ignored. Suppose you are creating a DTD to exchange
construction plans among builders. Because you have an international
customer base, you build a DTD that can handle both English and metric
units. You might define two parameter entities:
<!ENTITY % English "INCLUDE">
<!ENTITY % Metric "IGNORE">
