Conditional Processing Attributes
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W3C Reference: http://www.w3.org/TR/SVG11/intro.html#TermConditionalProcessingAttribute

A conditional processing attribute is one that controls whether or not the element
on which it appears is processed. Most elements, but not all, may have conditional
processing attributes specified on them.

W3C Direct Links
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* `requiredExtensions <http://www.w3.org/TR/SVG11/struct.html#RequiredExtensionsAttribute>`_
* `requiredFeatures <http://www.w3.org/TR/SVG11/struct.html#RequiredFeaturesAttribute>`_
* `systemLanguage <http://www.w3.org/TR/SVG11/struct.html#SystemLanguageAttribute>`_
