CSS selectors — :first-child, :last-child, :only-child

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div#t1 p:first-child {background: lime; color:#fff;}
div#t2 p:last-child {background: lime; color:#fff;}
div#t3 p:only-child {background: lime; color:#fff;}
div#t2 span {border: 1px solid #000;}
div#t2 span:only-child {background:blue; color:yellow;}