{{msg-mw|Usercssjsyoucanpreview|notext=yes}}

Should use {{int:Showpreview}} for the button label text. Also, the usage of single quotation marks in English is rather uncommon. Similar happened within Userjsyoucanpreview.

This is a message only used in MediaWiki 1.15 and possible in earlier versions and no later. It will not be changed.