The immediate response from browsers was to disable shared memory across JavaScript contexts (previously you were allowed to create a SharedArrayBuffer and share it across WebWorkers, which if you're familiar with UNIX you should read as "mapping a shared memory segment in multiple processes" or "threads with shared memory") and to disable high-resolution timers, which makes it hard to get useful information out of timing attacks. I believe SharedArrayBuffer is coming back now-ish and I'm not totally sure what the mitigations are.