Where do these horizontal lines in printed PDF file come from?

[pstein]

Lets start with an example:

https://www.thewindowsclub.com/best-free-internet-speed-test-apps-for-windows

As you can see (in Firefox or Chrome browser on Windows) there are NO horizontal lines visible on webpage (article).

Now I print this webpage into a PDF file with some available pdf printers (like wide spread PDFcreator tool).

Surprisingly the PDF file output contains lots of such horizontal (and vertical too) lines inside the article.
Mostly after paragraphs.

Why?

I guess there are additional, hidden CSS statements in "@media print" section.

Can this be?

Where can I find the responsible CSS statement for inserting these lines?