At first glance it could not be seen that they were doing it, but the consumption of computer resources increased when opening such web pages or having an extension with those intentions installed. Then an extension was created to prevent websites from undermining cryptocurrencies, although it did not work with other extensions.
It is actually possible to prevent websites from mining cryptocurrencies by simply adding a list to ad blockers, such as Adblock Plus. The list comes from NoCoin, now integrated in Opera 50.
As we all know, for some versions the Opera browser incorporates an ad blocker. After its most recent update, it introduces the NoCoin list as a recommended option.
From GitHub you can find out how NoCoin works and what are the addresses that it blocks to protect the user from cryptocurrency mining in the browser.