I came across some pages, e.g. https://www.autohotkey.com/docs/Variables.htm#Operators, where web clipper is not able so fetch the marked part of that site.
I tried to capture the marked part of page mentioned above and is visible in this sreenshot.
The result in Joplin 2.5.12 is in the following screenshot.
This site has a menu on the left which you can switch on and off by clicking on the three lines on the left side of the menu bar at the top.
If you try to clip the whole page the result is such as the following screenshot.
By trying "Clip simplified page" the result is:
None of the above is really satisfying.