Constraint validation rules are widely supported in browsers with the default behaviour to show an error form submission.
There was prefixed rules in Mozilla and Chrome to impact these however there appears to be no approach to standardise the appearance of these validation messages.
I’m struggling to find enough resource to suggest how browsers should actually show the messages, perhaps it is just a convention.
Would it make sense to allow the CSS author to use the newly resurrected appearance property to remove user agent styles and mint a new pseudo element?
In general styling of form controls is not standardized since nobody is super clear on how it should be solved. It is one of the bigger recurring problems. http://dev.w3.org/csswg/css-forms/ has some thoughts on the matter.
As replaced elements can’t take advantage of ::after users really need this so that they can style error messages whilst using native browser validation. Adoption of pattern and other HTML5 validation is very low and I think it is because of this.
So :user-error is useful however it doesn’t really help us with getting the error message into the label element for use with ::after, if that were the case then it would be only required to provide a way to hide native errors rather than change their appearance.