Enclose keywords in double-elements

I would like to kindly ask you to please take five minutes of 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. Any help would be very 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 keyword</mark>

Code (markup):

The function responsible for this job is...

Enclose keywords in double-elements

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