I would say same as Brendan or John by viewing this pictures.Looks perfect for a YBW from this view.
When do you mention no dark fringes to the tertials, do you mean no PALE fringes?
Was the bird calling? Is there any others pictures, from other angle, with only this one will be hard to do anything else from this bird. A picture with a better light on the wing, to see the shaddowing pattern could help i think.
I don't think this bird is one of the reguloides nor plumbeitarsus.
From this angle I think Chinese Leaf can not be ruled out and could match your description but you haven't mention any pale rump or median coronal stripe.
Actually I saw one this morning in Guangxi, this should be the second record for the province after a singing bird i found in april this year. I found the pale rump not very easy to see especially in bad light condition with a bird seen from below, it often can be only seen when the bird is flying, but the bird I saw this morning was very vocable. I found this morning bird a bit bigger than YBW and a bit more bulky. The call it was doing was very different from the bird i've got this spring, it was a kind od very hight and dry "tsi" reminding a bit one of the call of scarlet-backed Flowerpecker and sometimes followed by the same stroph repeated very quickly and decrescendo like "tsitsitsitsitsisisi".
I hope this can help.
All the best,
Jonathan
