They are so different that it isn't really worth comparing them.

Binding an event handler to the load event will call that function when:

• The load event fires (which is after the page and all its non-async dependencies have loaded) and
• The event loop isn't busy with something else

Passing a function to setTimeout with a time of 0 will call that function when:

• 0ms have passed and
• The minimum timeout for the JS engine has passed and
• The event loop isn't busy with something else (which includes the script which called setTimeout in the first place