It depends on how these words are being used.
If you say "Its being..." then being is a gerund and its is the possessive case. its and the words that follow it form a noun:
I was happy about its being a well established rule.
It sounds a little strange with its, but if I gave another example using another possessive pronoun, his then:
I was happy about his being accepted.
If you say "It being", then being is then a participle describing it:
I was happy about it being a well established rule.
And with another example:
I was happy about him being accepted.
There is a subtle difference difference in focus when using one or the other. I can best show what I mean with this example:
We were amazed by his singing.
We were amazed by him singing.