Selector {Declaration}  	H1 {color: red}  	Any H1 tag used in the site (<H1>)


Selector.Class {Declaration} 	H1.Alert {color: red}  	Any H1 tag that uses the class name of Alert (<HI Class="Alert">)
-OR- 	-OR- 	 
.Class {Declaration} 	.Alert {color: red} 	Any tag that uses the class name of Alert (<XXX Class="Alert">)


Selector#ID {Declaration} 	H1#Alert {color: red}  	A H1 tag that uses the ID Alert (<HI ID="Alert">)
-OR- 	-OR- 	 
#ID {Declaration} 	#Alert {color: red} 	A tag that uses the ID Alert (<XXX ID="Alert">)

More details read here

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