Disclaimer: I fully admit to a bit of obsessive-compulsive disorder on this.

The PHPEd CSS parsing mechanism is still missing several selectors, specifically "details", "menu", and "summary" (that I'm aware of), with no means of adding them to the CSS 3 reserved words list as a selector. I did try adding them under HTML 5.0 as reserved words but this had no effect on CSS parsing (..longshot, but had to try).

Any chance of future releases to add selectors to CSS 3.x? Considering all the (relatively) rapid changes that seem to happen in CSS and HTML, would help me to not see little red marks in my code margins

Agree. Even try to use the not so new element tag "svg" and it will be marked as unknown element. However, I think all those are validated by the outdated bundled HTML Validator. There is another thread requesting upgrade the validator to the newest possible available version. However there is no confirmation that will work or even better buying the commercial version will improve those validations.

At the moment I can add new CSS properties in the settings but not new element tags.