It means game over for users who run browsers like Safari, which don't isolate each site to their own OS process.
If you load Javascript from one site, that JS can read the entire state of memory for another site, if it is within the same OS process. This means that any site can include some nefarious javascript that reads all the cookies and passwords for the user on other sites, and then log in as them.
If you load Javascript from one site, that JS can read the entire state of memory for another site, if it is within the same OS process. This means that any site can include some nefarious javascript that reads all the cookies and passwords for the user on other sites, and then log in as them.