ifdef
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Content between the ifdef and endif directives gets included if the specified attribute is set:
<div class='example'>
ifdef::env-github[]
	This content is for GitHub only.
endif::[]
</div>
The syntax of the start directive is ifdef::$attribute[], where $attribute is the name of an attribute.
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Keep in mind that the content is not limited to a single line. You can have any amount of content between the ifdef and endif directives.
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If you have a large amount of content inside the ifdef directive, you may find it more readable to use the long-form version of the directive, in which the attribute (aka condition) is referenced again in the endif directive.
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ifdef long-form example
<div class='example'>
ifdef::env-github[]
	This content is for GitHub only.
	
	So much content in this section, I'd get confused reading the source without the 
	closing `ifdef` directive.
	
	It isn't necessary for short blocks, but if you are conditionally including a section 
	it may be something worth considering.

	Other readers reviewing your docs source code may go cross-eyed when reading your source 
	docs if you don't.

endif::env-github[]
</div>