When you add a new 'contribution' via utopian.io , the invalid tags such as those starting with numbers are allowed, and these are saved into steem blockchain.

However, when you re-edit the post in steemit.com, it will not allow you to save unless you remove those invalid tags. For example,

How to reproduce?

1. Add a new contribution in utopian
2. Tag something invalid, like the following:

Posts should not be accepted if they contain invalid tags.