Need to understand some thing

I have some css code regarding clearfix can somebody explain the code for me

for instance clearfix:after what is the : for also html[xmlns] what is the brackets for.  Is there some documentation online regarding  this.  

.clearfix:after {
	content: ".";
	display: block;
	clear: both;
	visibility: hidden;
	line-height: 0;
	height: 0;
}
 
.clearfix {
	display: inline-block;
}
 
html[xmlns] .clearfix {
	display: block;
}
 
* html .clearfix {
	height: 1%;
}

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