Browsers seem to support it fine, but the W3C CSS validator ( https://jigsaw.w3.org/css-validator/#validate_by_input) fails. The W3C HTML validator is also fine with emojis.
👤 Komodai Accepted Answer ✓
You haven't clarified you mean by "valid", valid to who/what? what is classified as valid?
Valid to W3C CSS Validator, no since you answered that yourself.
Valid to W3C HTML Validator, yes, you also answered that yourself.
Valid to browsers, yes, you also answered that yourself.