I think I have found a sort of work around for the OP question - instead of a transition beginning 'on.load' of the page - I found that using an animation for an opacity fade in had the same effect, (I was looking for the same thing as OP).
So I wanted to have the body text fade in from white(same as site background) to black text colour on page load - and I've only been coding since Monday so I was looking for an 'on.load' style thing code, but don't know JS yet - so here is my code that worked well for me.
#main p {
animation: fadein 2s;
}
@keyframes fadein {
from { opacity: 0}
to { opacity: 1}
}
And for whatever reason, this doesn't work for .class
only #id
's(at least not on mine)
Hope this helps - as I know this site helps me a lot!