Dress code in Italy

Just make sure your knees and shoulders are covered

My family traveled to Rome in 1995, and the clothing police in St. Peter's Cathedral took issue with my 13 daughter's knees. She was wearing Bermuda style shorts and a t-shirt. What to do? We walked down the street to a street market and bought her a long skirt with an elastic waistband that covered her legs completely. I think it cost $5. Back to the church, and she was good to go. After we left, she just pulled the skirt off and it packed into my day pack slick as you please. I don't know how they would react to Lululemon yoga pants, particularly if they are flattering on you.

The rule is shoulders and knees covered, nothing about skirts or dresses. So pants or Capri pants and a short sleeve top are fine. I’ve worn leggings and a sweater before, no issues.

If you are very concerned then have a scarf in your bag to wrap around you as needed (also good if your top is sleeveless).

I don't know what you exactly mean with attractions, but only churches and church owned museums (i.e. the Vatican Museum) enforce the dress code. Nobody cares how tourists dress in state run museums (like the Borghese Gallery) and public parks (like the gardens of Villa Borghese) if they aren't half naked.

In churches nobody cares how you cover your knees: you can use a skirt, capri pants, a scarf or a plastic bag. It's about modesty, not fashion.

Out of curiosity, if you see a sign with cleveage restrictions would you take a picture and post it here?

As long as your shoulders and knees are covered there will be no problem.
One person's opinion of what looks good or fashionable is not in question ; it is just a sign of respect to not have exposed knees or shoulders.

Though, if you are famous exceptions will be made (google Jennifer Aniston Vatican to see what I mean) and in general for non famous folks leniency on the rules is sometimes extended to boys in shorts but rarely girls/women. Even if you get past the guards it still disrespectful to not follow the rule.
For very young children ; babies/toddlers these rules do not apply ; they are seen as too innocent in the eyes of the church.

Don't get your hopes up too much about the gardens of Borghese. Think much more fairly dry city park than gardens.

Don't try this yourself, please, but last fall, in Florence, in several churches, I saw quite a few Italian teenage girls wearing long pants and long-sleeved shirts EXCEPT there were huge, intentional holes at both shoulders and both knees --- completely exposing these body parts. I was longing to ask someone who knows about such things how these fashions fit into the church dress code.

Curious did you see them inside the Duomo in Florence?
Of all of the churches in Florence, that is the only place I saw security / dress code enforcers and when I was there they indeed were denying entry based on dress and seemed fairly strict.
The rule technically applies to all of the churches but only the very largest of them has the money to pay for guards to enforce the policy.
It almost always seems to be a teenager daughter with her family that is not dressed enough, the parents arrive clueless about the rule, etc... they often go buy a scarf, wrap it around the offending area and problem solved.

You mean the young ladies with the holely but not holy clothes? No, not in the Duomo because I wasn't inside the Duomo on this trip. But other major churches like Santa Croce.