you CAN send a pwm to an SSR..
but don't. For the love of god don't
Optically isolted relays require time to do SEVERAL things:
1. Hit LED on.
2. Hit LED off.
3. Photoresistors take time to hit on as well.
4. Take time to hit off.
1. Gate Current increases electron flows.
2. Gate current decreases electron flow.
Both of those are done to a point of either hitting it on/off at a RAPID fashion. We are talking thousands of times a SECOND. *Very* rapid. SSR's can probaly do the same, but its not the same.
MOSFET's are the standard for a reason.