"TemplateSandbox" lets you preview a page with a change in a template it
uses.
At the bottom of the edit form in the Template and Module namespaces, a
new box will appear that allows you to preview other pages as they would
appear if you saved the template/module you are currently editing.
Example: try editing the sample "thank-you" template
https://www.mediawiki.org/wiki/Template:Thankyou . On the edit form,
under the Save button, you'll see "Preview page with this template" --
try User_talk:Example_user .
Or, go to
https://www.mediawiki.org/wiki/Special:TemplateSandbox
and give it the "sandbox prefix" User:Sharihareswara (WMF)/sandbox and
tell it to "render page" User_talk:Example_user . You'll see the ugly
changes I made to the template in my sandbox.
More information at
https://en.wikipedia.org/wiki/Wikipedia:VPT#New_feature_needs_testing .
You can check this out in the new version of MediaWiki, now live on
mediawiki.org, but until we have more feedback and improvements we
aren't pushing it live on the other sites.
We figure this will make it easier for people to hack on templates, and
to develop new Lua templates to replace wikitext templates.
--
Sumana Harihareswara
Engineering Community Manager
Wikimedia Foundation