OK. Now I got it to work.
I can't really say that I used much of your last instructions.
I did some more searches, and I found this:
http://stackoverflow.com/questions/3272071/iframe-to-only-show-a-certain-part-of-the-page
There I found the following single JS line that uses jQuery:
$('#target-div').load('http://www.mywebsite.com/portfolio.php #portfolio-sports');
Then I started reading the W3schools.com JS tutorial, and when I got to "JS Output" I was enabled to 'concoct' these 2 pages:
http://www.transformation.dk/iframeexample.html
http://www.transformation.dk/LOCALIframePage.html
So there might be an even more elegant solution, or maybe a very experienced JS programmer can point out some weaknesses in certain browsers etc., but I only use that single JS-line mentioned above! Everything else is just HTML and CSS. And even <iframe> became obsolete and has vanished from the above mentioned "iframeexample.html"!!
So thank you very much for your help!
Best regards,
H.R.