ID Selector

            CSS:
            <style>
                body { font-style: normal; }

                #opacity { opacity: 0.5; }
            </style>

            HTML:
            <p>Normal font-style taken from the body style</p>

            <p id="opacity">P text with opacity</p>
            

Result:

Normal font-style taken from the body style

P text with opacity

Important: ID is unique in page!!!