Enclose keywords in double-elements (please, help me!)

I would like to kindly ask you to please take five minutes from your time to help me with the code below. My JS experience is very limited and I have been struggling for hours to get this done, without any success. Coming here and asking for your help is my last resort, and any help would be so much appreciated.

I am using a JavaScript that highlights certain keywords on the web.

For this, the script encloses these keywords within a '<mark>' tag, like this:

<mark>highlighted...

Code (markup):

Enclose keywords in double-elements (please, help me!)

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