How limited is support for Level 4 in Safari Content Blockers?

Question

Back in 2015 when Safari Content Blockers were released it was said that they support CSS Selectors Level 4 expressions.

However even today they don't support at least the :has() operator.

What parts of Level4-only specification does Safari actually supports?

Is this the only what is supported (per Webkit blog post):

:matches(), :not(), :nth-child(), and :nth-last-child()?

Answer 1

Safari 9, released in 2015, shipped with support for the following level 4 selectors:

• Case insensitive attribute selectors, e.g. [type="text" i]
• :any-link
• Level 4 :lang()
• Level 4 :not()
• :matches()
• Level 4 :nth-child(An+B of S) (implied likewise for :nth-last-child())
• :placeholder-shown

As of 2018, no new level 4 selectors have been added since. However, Safari remains the only browser to support all but the first and last of these.

There are no known browser implementations of :has(), and frankly I wouldn't hold out on one surfacing within the next few years. Given that browser implementations are optimized for matching CSS selectors live, implementing snapshot matching is going to take a non-trivial amount of work which doesn't seem to be work vendors want to invest effort into right now. The WG is still exploring their options in the performance department.