These have been around for years ... but it's still a scam.
The cost to mine in your browser is probably 1000x the BTC you'd ever get.
Some sites, years ago, would add these on to anyone who visited the site - basically just as bad as a bot net - but even worse now coz it will make absolutely nothing.
100MHs will currently expect to take ~65 thousand years to find a block ... if the difficulty never rose ...
what if this was promoted on autosurf traffic?
Would you include the truth prominently on the web site?
That truth being:
"Hey everyone who visits this site:
If you are using a mobile phone, we'll run your battery flat real fast and generate next to nothing even if hundreds of thousands of you do that.
The great mobile phone bot net that doesn't even make the bot net owner anything.
If you are using a desktop computer, we'll run up your electricity bill by about 1,000,000x the amount we'll make out of you.
Lets hope we can get together a 1,000,000 PC bot net so we can make a few $ a day."