Language
HTML & CSS
Structure and cascade: semantics for machines, constraints for layout — not a programming language.
HTML is a document language. Semantics (headings, landmarks, forms) are an accessibility and SEO contract. Div soup is not “just presentation”: it is lost meaning.
CSS is constraint-based: cascade, specificity, inheritance, then Flexbox and Grid. The particularity is that the browser is the runtime — you describe rules, you do not paint pixels. Utility frameworks (Bootstrap) speed layout; they do not replace understanding the cascade.
A senior PHP who cannot read a stylesheet will ship Admin-looking fronts and inaccessible forms.