The meaning of the sentence is changed slightly by the placement of strictly. I think the best way to phrase the sentence so it is clear and unambiguous would be to say
... the reference format has been improved by according strictly to the IEEE standard.
if you mean that the improvement happened by means of according closely to the standard. "Has been" sounds better than "is", because the action of improving it happened at an unspecified time before now, and it matches tense better with the have polished earlier in the sentence.
Let's look at some possible word orders and how they change the meaning slightly.
...strictly improved according...
implies that the improvement was strict. This is probably not what we mean.
...improved strictly according...
implies that the improvement was only in the opinion of the standard of the IEEE.
But if we use by according strictly, we give the sense that the improvement happened by means of according.