Fix Live CSS with an iframe element

For #7935 and #7785 

Bugs to fix:
- [x] Only updating the iframe if the same CSS file is used for both iframe and page
- [ ] Changes to the iframe's HTML will only take effect on save
- [ ] Error message on startup (in console)
Yes, that's how the array is structured.
I don't need to have the first entry, AFAIK every entry should have the same value.