Explain a CSS style

A page has been downloaded with a style as follows:
#slinner div#slidenum DIV.slideNumber:unknown {clear:both;   Display:block;   Visibility:hidden;   height:0px; Content: "."}

I don't quite know what it means.
#slinner Div#slidenum Div#slidenum:unknown What does this mean,
Div#slidenum Div#slidenum also: What is the use of unknown respectively, and the content of the style: "." What is the meaning




#slinner表示id =slinner
Div#slidenum represents id=slidenum and must be a DIV element
Div#slidenum:unknown says Id=slidenum and must be the unknown underneath the DIV element, it's probably just a style without content.
It's similar to clicking on a super connection that has not been clicked.

Note that a space represents a level, that is, the Div#slidenum is already the second level, that is, it must be included in the first level #slinner inside.
Your example will be applied to the example
<div id= "Slinner"
<div id= "Slidenum" >
<div id= "slidenumber>
</div>
</div>
</div>


A:link {}
a:active {}
a:visited {}
Similar to this, this means that the connection has not been accessed, the activation status is CSS style

That unknown and this should be the same, unkonwn should be wrong or no content when the style
SlideNumber should be a pseudo class bar ~