This is the new trend now , profitability with GPU-s is going down day by day and people build all new kind of websites ,some gambling related, some games for PC related, some torrents sites and they use our CPU quietly without telling us. If they use a lot of CPU-s sure that is a great way to mine monero, bytecoin or any other coin that can be mined with CPU and to make a lot of money with little work.
I have seen few sites like free live TV and free movie streaming sites having hidden browser miner on their site, when people will focus more on movies/serials or channels they might not notice the loud sound CPU produces whenever those browser miner will start.
Easy way to find out whether site have hidden miner or not it is to check your task manager, if your CPU consumption is near 100% than the site which is open in one of your browser have inbuilt browser miner.